I am doing a SQF garden in our community garden but the plots are only 4x8 and raised beds. What I would do if I were you was just mark off 4x4 beds inside your plot and plant accordingly in the ground. Yes, you don't get to take advantage of the raised beds for their benefits of less weeds, better draining, and warmer soil, but you do get to take advantage of the space saving. Mel's mix is great but a lot of gardners plant in there native soil and yours has already been ammended with compost. I say just treat your plot as a non raised bed square foot garden. If you really want to make an authentic Mel's SFG, make a little one at your house this year and see if you like it better than the community plot. Then you will know for next year. Remember it is only gardening and have fun with it.

As far as raised bed material, you can use anything from cinder blocks, bricks, large rocks, to natural rot resistant woods like cedar.