I'm slowly building out basement gym equipment, either purchasing when it made sense (affordable stall bars, for example) but I couldn't bring myself to pay hundreds of dollars for a plyo box. There are a ton of DIY plans to find online but I liked this one because it laid out the cut lines for a single 4x8 sheet of 3/4 plywood. I paid $55 for birch from Home Depot and I couldn't be happier with the sturdiness. Works great as a pommel horse if I put a mat on top of it to do hanging leg extensions.
Have fun!