plywood tank project

hello im new! im building a plywood tank. dimensions are going to be 42''x24''x30''. Im getting help with more carpentry savvy family but still messed up measurements due to not knowing 2x4s arent actually 2x4 so now ill have to add a bit of wood above them or bring the height down to 29 inches...

I am planning on building a bigger plywood tank in the future, but I was thinking I could practice that by building a smaller one for now. Plus I like the idea of the compact look a little plywood tank would give if I painted it black. Is it even worth it based on the costs? Also what thickness...