I have some pods. They aren’t my favorite (due to the slow action) but they are probably more appropriate for the intent of a “budget” and “simple” build.
My basic idea was a 2’ x 3’ piece of plywood deck, with a 2" x 4" x 24" long board as the center “axis” and simple springs on the sides (tennis balls, balance pods or some other springy material). Maybe some retention straps to hold the trainer and that’s it. Dead simple and can be built with a screw driver.
I prefer a rigid center, compared to the floating effect that happens with pods, but those work too and keep it even more simple. I just dislike the “bounce” that happens with a full spring support (and no rigid center).