Rainbow Box

Haven't made one but I have experience building with curves and lots of other jibs.  First I would buy some 2x8s.  Cut two the same size to be the end supports(I'd say 8 inches) and one to be the middle support(14-18"). Make a sturdy frame with these (a 2x4 straight across on both sides should do).  Get some 1/2" or 3/8" plywood and cut it 1 foot wide and screw it to one of the end 2x8s, then the middle piece with just 1 screw for now.  Then stand on the other end of the plywood and secure it to that end.  If you like it, add some more screws and add the puckboard (I assume this stuff bends). Make sure to counter sink into the puckboard.

Coping is always a plus, but if you don't have it it will still be fine.

i would definitly use coping, otherwise you're gonna tear the shit out of the edges of the puck board.  but to make the box, i would just cut the shape you want out of plywood or osb and then make an identical copy.  then bridge 2x4s from one peice to the other.  this will give you width and the support for the top sheet

make a 2 x4 frame then bend some plywood over the top and then screw down the puckboard. If its a really sharp curve you can just use 2 thin sheets. Coping will def make the puckboard last much longer but its a little harder to bend.