The runners and two cross-braces/seat supports were fir 2"x4"s (or so my neighbor thinks, since I can't identify wood from looking at planks) from a torn-down shed. The front cross brace is left over from building my box. The seat planks are cut from a planed 1"x6" rescued from the highway by my wife (pitted by gravel on the other side) and 1"x3" from a small pallet.
Held together with doweling and screws. Finished with some spray-on outdoor water-based varnish. Waxed the runners with a candle. I still need to put a rope on it.
It's simple, but I think it looks good. Now we just need it to snow around here so he can use it.