Okey, I see that we were working together today!
You are doing a fine job, can't wait to see it finished. You've got a good feline companion, too!!
Is this a dual-hx housing? I see there is plenty of space inside.
I'd suggest you to use some aluminum foil to cover the inside of the box in order to create a useful UV barrier and save some more energy.
Beware of the water that will drip down the exchangers, as you said that kind of fiber board is not very keen of it. You could use a plastic collector underneath and drill a small hole for an evacuation plastic tube.
I tried to fix the first motor housing in place and drilled the exit hole. Was a bit tricky to calculate the optimal position, but i think I got it right.
This time I used some urethane foam sheets on the back side and neoprene tape for the vibrations
