Sorry, not a clue. Car audio is such a dead art that practically anything good has to be made as it is all geared to cater for muppets who want bright colours and noise
Last time I looked there were a few slot port 8s on eBay but not details about spec
My brother and I made a couple 12 inch sub boxes a few years ago for a big loud system for him to use at uni. With a couple jbl 12s, proper boxes and about 400rms each (ie not a lot by ice standards) its enough to be overwhelming in a large indoor environment and still does well outdoors too
Basically 12 inch subs in car are for show, unless you're an spl junkie.