I build all my boxes, boards, and pliths (russian word for wooden block that plates sit on for a cheap way to do rack pulls) from cull wood 2x6s from Home Depot.
For boards and pliths I just hammer or screw together the boards cut to the length I want. My board are usually a bit over a foot long and pliths 2 feet or so. The box I build by cutting the 2x6 into 13 inch lengths. I lay 2 of them "east-west" then overlay them with 2 more "North-south". The 2x6 are actually about 1.5x4.5, so you have about a 3 inch Gap in between the main boards. If you want you can cover the top with some thin plywood.
These boxes are strong, relatively light, and stable. Cheap too. I have a 9, 11 (1/2 inch plywood on top of 10.5 inch box), 13 (1 inch of plywood on a 12), and 15 inch box. Wide stance in Chucks my 11 box gets me to IPF depth, normal stance in RDC squat shoes I use the 13.
Cull wood is so cheap I build them an dleave them in 3 gyms i train in and have extra at home. Also good for bike workstands, painting kids toys, step ups, plyos, and car jacks in many rural areas.