You can use any wood you like for the box as long as it's unfinished and not pressure treated. Carpenter bees don't like finished or pressure treated wood. I happened to have some old barn wood that wasn't good for anything else and it made for some nice rustic boxes. You need 5 pieces of wood to build the box. The large sides are 7 1/4" wide at the top, 4 1/2" wide at the bottom and around 8" tall. Mark 7 1/4" on one side of your board, set your miter to 10° and make the two cuts angled towards each other. This will give you 4 1/2" at the bottom or pretty close. These dimensions aren't set in stone so you can adjust to fit your fancy. Repeat and you have your first 2 pieces.
The third and fourth pieces serve as the other sides. They are made from 3" wide wood and it's easiest to just rip some 3" wide boards before you start. I don't have the exact measurements for the length because it will depend on how tall you are making your box. Instead, take one of the trapezoid pieces and use it to mark the length of the side pieces. Your marks will be at angles oriented in the same direction. These are also 10° cuts on your miter saw and make the top and bottom of the trap flat. Assemble your box with some wood glue and nails. Don't get your nails too close to the top if you plan to slope the roof (read on).
Picture 2 - To slope the roof a bit I tipped my miter saw 10°-15° and cut the top as far as possible. I finished the cut with a hand saw. When assembling your box make sure to keep the nails an 1" or so from the top so you don't saw them.
Picture 3 - Drill a 1/2" hole angled upward in each side near the top. The bees prefer to climb up into a hole and it also makes it harder to find the hole when they want to leave.