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.