I have a nice 89 Mercedes 560 SEC coupe. Looks good for a 20 year old car and I needed to replace the speakers a few years ago. I don't need to spend thousands for a car system when I have way more than that in high end stuff at home so I only spent maybe $600 replacing the 4 stock speakers, adding two 8 inch subs and a modest 4 channel amp. I had a guy mount the subs directly to the package shelf behind the rear seat and they just hang there in the open with no holes cut in the shelf. To get the best sound I should enclose them but like you, I need pretty much all the trunk space for for gigs and a standard box is too big. I would have to custom make something and measure it carefully to make sure I still have room and I haven't done that yet because those subs sound pretty good as is. I get plenty of bass right though that shelf. Of course, I listen to regular music not strong boom boom stuff and this is all very subjective. You may well want better sound than I do, I really don't care about a great sound in the car, just decent like a stock BMW system a friend of mine has.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.