Welcome Wendy! I also own a Play Pac. Mine didn't have the interior lining. You could use automotive or marine headliner. Due to allergies, I'm not putting headliner in mine. It was painted before and looked good. The people of this forum are great with advice and help.
BTW, the originator of the Play Pac is a member of this forum.
Also, we like pictures around here. So, if you have any, please post them.
Plotting my next adventures...