Mason bee nest boxes are easy to build and other than maintaining the hole size of 3/8ths inch, stock doesn't much matter. You can use any scrap you have laying around. If you are drilling holes side by side try to leave 3/4" to 1" between holes.
Side note on hole size:
To avoid confusion use a 3/8 bit for blocks with paper tubes. Use 5/16 bit if you are just drilling the blocks and not using paper tubes. I recommend the paper tubes to avoid sanitizing the blocks each year. Here are links to the bits. 3/8 by 12-Inch or 5/16 by 12-Inch Drill Bit
Here is how to build your block.
You can copy and save this video to your computer using WM Recorder
How To Build A Mason Bee Block from DavesBeesvideos on Youtube.
In the video I used a 2 by 6 cut 8 inches long. A standard drill bit will only reach a little over 4 inches. A 4 inch hole will work but you end up with less females as the bees like to lay female eggs in the back of the nest. To stay with the instructions in the video you need to buy a 3/8 by 12-Inch Drill Bit or slice off the back thick enough for a standard drill bit to go all the way through.
Another option is to drill straight in as far as the bit will reach without cutting off the back and eliminate the paper tubes. For a great little fact sheet on simple mason bee blocks check out this Free Publication Fact Sheet 301 from the University of Maine Cooperative Extension.
If you have checked out the KTBH videos you know that I really like this Black & Decker Firestorm 24 Volt 4-piece Cordless Combo Set You can literally build the Mason bee blocks and the KTBHs with this set....I do it all the time. There is also a Black & Decker 18-Volt 4-Tool Combo Kit that is cheaper but should work just fine. I also recommend the 24 Volt Dual Charger.
The block in the photo on the left is more like 5 by 6 inches and is drilled all the way through. It is filled with paper tubes and a scrap board is used as the back.
The only real difference between a raw drilled block and a block with tubes is the reusability. All the instructions apply except the entire block is stored for the winter and the entire block is put inside a release box in the spring. You just need a cardboard box big enough for the bees to emerge from the block and make their way to the 3/8 inch hole you put in the cardboard release box. For this size block a shoe box would probably work great.
For instructions follow this link. Mason Bee Nest Block Instructions