the box is unnecessary.
my previous garden was an acre of raised beds... and not a board in sight.
It's always a good idea to garden in a spot for a couple of years before building anything permanent, anyway.
the important part of gardening is amending the soil after digging the turf out.
even that can be done on the cheap.
Whether using your own [& the neighbors] grass clippings, or going down to the local riding stable for horse poop, it can all be had for little or nothing.
One time... I was at the horse barn getting poop, and this college girl was there with a volkswagon convertable (bug), and plastic garbage bags... I helped her fill them, and even loaded them for her...
Anyway, where there's a will, there's a way.