I would use caution if building them there.
moisture will find it,
and mess up your mdf.
and maby some one will step on your pod, or speaker, and wreck it.
I actually took apart my doors to find a good spot to cut in some 6.5" holes
for speakers there. found out that I could fit in a pair of 8" or a set of 6by9 -
but I kept the 6.5" speakers since I already had them to use. but now I can upgrade to some 8" if I find them.
for my doors. alot of room in there foward up. all my window motor stuff is dead center of the doors.
give em a good looking over to see what space is avalable.
take off a door pannel, and run the window up and down a couple of times
to see wherere all the crap inside is running to. lets you know how much space is avalable to work with. migt be suprised how much room you got in there.