What you might consider is making several small sized boxes or blocks that can stack. If you make 5 boxes sized 1", 2", 3", 4" and 8" you would have the ability to create any height between 1" and 18" in 1 inch increments. The boxes can be used for much more than box squats. You could use them for deficit deadlifts, stepups, calf raises, depth jumps, pulls from blocks, etc.
Also, a simple way to create these it to use 2x4’s with a plywood top. I like an 18" square size. You can simply cut a bunch of 2x4 into 18" long pieces and use 3 per level and stack the next level at a 90 degree angle. Use screws to connect them and top it off with a plywood top and a thin rubber surface to prevent then from sliding.
Getting the plywood cut to size may be the most difficult thing if you don’t have the tools however the big Home Supplies stores can usually cut them for you for a small fee.