Some cheap high-gauge sheet metal would work just fine. The same stuff that the ducts are made of. The S1s even have nice edges from the metal mounts to flush up against. Any supply store that has heating/ac equipment should carry it and sheet metal is pretty cheap and bends super easy. It also can cut your hands super easy. Duct tape corners and hard edges. 

4" and 6" duct are pretty common sizes. Depending on your location one size may be easier to find major accessories for than the other without going to a specialty Heating/AC store.

Another thought is for like an extra $10 you might be able to get a specialty Heating/AC store to pre-bend a plenum for you if you give them the rough dimensions. Or you could just oversize (incase you get more S1s) and go with an off-the-shelf size. Make sure its not one with insulation on the inside. Or you'll have to rip it out so you're S1s don't catch on fire.