me and a couple friends built a few boxes this summer using wood frames. we did use small metal rails for the coping, but they were easy enough to drill through with a powerdrill. also, we ordered polyethylene online for the top. it was a bit pricy but worth it, the stuff is slippery as fuck.
we did all of this with almost zero experience and ended up with a couple of solid boxes. it didnt take long either.
sorry i have no rail-building experience but i can give u more info on how to make these boxes if u want.