Mounting depth is sooooo not a problem. I am leaning towards the fiberglass tub as well... maybe backing the tub with thick foam will provide a pressure fit against the door? I am still trying to work out how to mount an MDF baffle to my door opening and have it be secure. I've got 5 gallons of damping at my disposal, so vibration shouldn't be a big problem.
Here is my idea so far... This is basically what the door looks like without the cover.
And this is how I think the baffle will work with the door. It is built out some where the woofer sits, and I can take it out a total of 3 inches if I need to. I really don't see .3cu.ft. being that hard to fit in, but then again things rarely are as they seem...
Any more ideas? Man, after I deaden everything and build these pods in, my doors are going to shut like a vault from all the weight!