A few things you can do...
You will have to spend something, so bite the bullet, and open the purse.
Pre-cut 3/4" lumber comes in 2 1/2", 3 1/2", and 5 1/2" widths. Wider, but I think this is in your range.... determine the sizes you wish to make....Think of them as picture frames. Carefully draw the frames, showing joints so you know the length of each piece. If it is square, then two pieces will be 1 1/2" longer than the other two pieces, allowing for the overlaps. When you know the size box you have, you now need to make a bottom. The bottom will be the exact outside dimension of the frame. You now have a cutting list of materials. You can go to Home Depot, and they will cut plywood or OSB (oriented strand board, which is cheaper than plywood) (3/8" or 1/2" will probably work) to your exact sizes.
I would keep the sizes of the boxes in increments of 8" 12" 16" or 24", as this will allow you to design the sizes with a minimal amount of waste. They will also make the cuts for the linear material (sides) if you provide them a list. Unless you use cedar or redwood...this will only last a season or two, as the cheap lumber will rot.
You may also try old pallets or skids....some of these are made of hardwood, but sounds like you are not handy enough to cut them up... As a last resort, try various construction sites that have dumpsters. You will be amazed at how much wood is thrown out. But most of this will require the removal of nails.