I am trying to fit some equipment in a tiny space and the mixing box that the manufacturer sells has a 34"x5" opening on it for the outside air intake. I don't have anywhere close to enough room to accommodate a transition piece to this size and was wondering if there is typically anything special about the mixing boxes that manufacturers sell with their equipment or if you are just as well off ducting the equipment and making your own "mixing box". Then I can make all of the openings where I need them rather than being stuck by the limited variations that the manufacturer offers.
A mixing box just mixes inside and outside air to equalize temperature. It's possible that using a smaller box may reduce its effectiveness somewhat, but in general you're simply transporting and mixing air, so my unsupported opinion based on no facts whatsoever is that your solution should be fine.