OP: You should definitely try these kind of questions over at DIYaudio. Lots of builders there with hands on experience.
How properly covered internal sides of enclosures for speakers with a passive radiator?
My question to people who have practical experience to build a sealed (closed) cabinets for speakers with passive radiator?
I have experience to build speakers with ported cabinets, but now I've decided to build a three-way sealed cabinet using one woofer in the middle of the front panel and one passive radiator below the woofer. When I built the cabinets with ports, to minimize reflection and etc, I put felt material on all interior panels : left, right, rear, bottom and top panels (except the front panel). I know, passive radiator works using air pressure from woofer.