Mason bees seem to need a certain diameter hole, about 5/16". They do not seem to be too particular as to what that hole is in. I have seen them fill holes in my cedar siding and some bolt holes in my garage door. I took a soup can and stuffed it full of drinking straws from McDonalds (their straws are larger in diameter than most places use) and they seemed OK with that. I have also drilled holes in 4x 4's and 2x4's. Some people set up a table saw with dado blade to cut slots like those shown in the picture you referenced.

Mason bees are more effective at pollinating fruit trees because they will work in colder temperatures than honey bees will. If you have too many bees you might have too much success and have to do more hand thinning of the fruit!

Last year I think my bees suffered a lot of predator damage. I had the blocks set near a wire fence where sparrows could sit and pick the bees off as they returned to the blocks. This year I will put the blocks inside an enclosure with 1" chicken wire that the bees can fly through quickly and then land on the blocks where it is safe from predation.

I put a mound of dirt in a shallow pan of water near the bee blocks so they don't have to fly too far to get mud to plaster the cells shut. I use a water heater pan. A friend of mine leaves plastic buckets of water around his yard for his bees but the bees drown when they land in the water and can't crawl up the slippery sides of the bucket.

How many mason bees do you need? Hmmmm. Maybe none at all. Then again, maybe there's plenty of them around already. Do you have any pollination problems? Of course you won't know what good raising mason bees will do until you do it.
If your 20 trees are truly close together then I would assume the area covered is less than 2500 square feet. Seems to me a dozen working females would be more than adequate. If you're buying tubes then assume you'll get 2-3 females per tube. One nesting block with like 50 holes would be adequate. You might consider just putting out the nesting materials (tubes, blocks, bamboo, whatever) and see if any local mason bees move in. Although stocking with new bees would ensure a population.
Something to consider... you'll want your mason bees active over a long time to cover the various blooms you have. If you have too many bees, they will too quickly fill up the available nesting sites and move elsewhere. And since their poplulation will increase every year, you'll have more and more bees every year. My solution to this is to get rid of literally hundreds of filled tubes every year by donating to the HOS, giving some away, while some unfortunates end up in the fireplace. Ouch. The point is that we are able to give them such ideal living conditions that their population will get higher than you will be able to accommodate with nesting sites. So they may leave too quickly unless you thin their population each year.

Here's a picture of a couple of my mason bee tube houses, at the bottom of this page:
http://www.homeorchardsociety......rticle/23/
Each hole you see has a cardboard tube inserted.
Made of cedar with a copper roof. Defininately overkill but I do it for the fun, although shelters like this are extremely useful, practical, durable, and look cool too.
You can cram a bunch of tubes into a milk carton with one side cut out, or any such sort of container.
But, yes, I would keep them out the rain. Water is bad news for these guys.
They should face *south* or *southeast* to pick up the any morning sunshine.
I wouldn't bother providing them with any mud, they seem to find it themselves even in dry weather. But if you want, you can just dig a 12-inch deep hole, fill it with water and let it drain. If it gets dry, just refill it at *night* some time so it has time to drain before the bees come out in the morning.
Attaching the bee shelter to the side of a house is often a good place. High enough that it gets protection from rain under the eves, but low enough to get morning sunshine and is convenient to work with or look at. If you simply have *no* shelter place to put it, then a simple shelter can be build. Or build something with a substantial overhanging, sloping "roof". A wooden box can be build with an open front and a sloping roof in the form of a chunk of 1x8 cedar. Just make sure that the roof overhangs the front enough to protect from wind-blown rain. The shelters in the picture in the link above were designed to be hung on a fence with no overhead protection. The worked great. There's a little metal attachment you can see at the top/back where they can be hung on a nail.

There are several choices for filling the back of cardboard tubes.
The ones ffrom KnoxCellars.com come with a plastic plug. You can order plugs from them if you like and if they'll fit.
I also have a bag of corks that I ordered from a cork manufacturer. I forget the correct size designation that fits.
Or you can leave them open. Probably better if you push them back against the rear wall of whatever container you're using. The bees will seal it off anyway.
I've read that Mason Bees would not nest in open-ended tubes. Not true. Perhaps they prefer closed-end tubes, but they certainly use the open-ended ones without problems. Like I said, they'll gladly seal the back end themselves. And if it's longer than they care for, they just shorten it with a mud wall.

Yes, push the tubes all the way back. The bees will prefer it this way, they like a bit of shelter over the nesting sites.
Tilting the container to natually shed rainwater is also a good idea. Better yet, keep out of rain entirely. The tilt will not bother the bees, they will nest in a completely vertical tube if need be.