your only real competition would be from stealthbox
and their stuff is good, but hyped and mostly poorly produced and too expensive
when i was hunting for a solution i discovered quite quickly that unless you buy JL with suited enclosure then it was pretty much stealthbox or nothing for the UK market
<<---- if you peek at 'my ride' thread and look at mine (built for a 13.5" sub) i got that done and finished for £200 (just the enclosure) and i paid for materials
sounds cheap but thats only because i had other stuff done so when totalled it worked out better than done individually and cheaper than stealhbox too
i'm waffling i know, but personally i think if you offer it with size to suit (8/10/12/13.5 etc) and done acurately for volume then you could be onto a genuine money spinner
as long as you make sure you dont foul the parcel shelf - worth noting here that the ball on the parcel shelf string when the boot is shut acts like a wrecking ball if the sub is not protected, i tuck mine into the net on the underside of the shelf but will be retro fitting MK4 golf retractable strings to solve it.
and that the boot carpet can still be lifted if needed too
long story short, offer the sizes, carpeted, with photo fit instructions and you will rule ebay buddy
should have just said that from the start