Here are some pictures of my progress on an enclosure using a Scan Speak Revelator 28W/4878T00 beneath the storage lid of my 2016 X6 F16.
I opted to leave the liner in. My job would have been easier if I took it out. The goal was to get as close to 1.5 cubic feet sealed while leaving enough room for at least one amplifier. There is also a lot of room where the stock amp is.
Madisound recommends a 1.5 cubic foot sealed enclosure for the 28W but says anything over 1 cu. ft. is acceptable. I am too lazy to do the math but I guess I ended up with around 1.35. I'll add plenty of stuffing. I was able to get 1 inch of clearance on the high side of the magnet but since the top slopes, a bit less on the low side.
I think I am close. I may need to sand down a few areas to make sure the enclosure doesn't touch any of the metal frame or secure it to the frame to minimize rattles. I was pretty nervous testing the first fit and needed to take some material off the back high end. A one inch round router bit did the trick. I may take it down a bit more with a sander. No issue since the top is a 1 1/2 inch MDF/Baltic birch sandwich. For some reason the high end doesn't fit flush to the floor.