Find a local cabinet maker, ask them to assist in construction or to donate. Small local businesses will utilize and appreciate the tax write off more than the local HD or other global companies.
Fwiw, a pellet trap does not have to be complicated. Any standard box made out of 1/2" osb and using hanging heavy carpet scraps (three or four straps hanging 2" apart) is more than adequate to stop .25 call pellets even. I imaging your group is using Avanti 853 guns which are relatively low power anyways, so no need to have sheet metal lining.
-Alexander
My candle burns at both ends; it will not last the night. But ah my foes, and oh my friends - it gives a lovely light.