Single/duel band pass sub boxes were all the rage back in the day, the ones where the sub is literally mounted out of view in the middle of a box. Are those not a thing any more?
Those fell out of favor for a long time but have been sort of making a comeback over the past several years. 4th order bandpass is the simplest to pull off but is basically a worse sealed box when you boil down performance. You can get some gain (ripple) but not really any place that helps for music listening applications IMO, and if you want one that plays a wide and flat response you're typically going to want to find a sub that will perform well in sealed alignment (which may be easier in the 600W and under power handling category).
6th order can give you good bandwidth AND gain but they're extremely finicky in getting right. You'd need to do your homework to make sure the specs of the sub will allow any reasonable looking response curve and then likely still plan to tweak the box/port as needed to dial it in. I've seen several of these turn out poorly, though on paper and some reports online do show good results for people who put enough time and effort in.
The other weak link about either is that it can become hard to hear/smell warning signs when you're pushing things towards failure points and then when/if you do break a woofer replacing it is going to be a bit less simple since it's buried/hidden in the box. I'd say they'd only really be worth the trouble if you were doing a trunk porting up through a ski hole or up through a rear deck, a blowthrough on a pickup truck, or similar where you are trying to get sound up into the cabin out of a vehicle which doesn't have a more open cargo area to mount a box.
If you have mounting depth issues on your wide side I don't see that sort of box helping you at all assuming we're talking about the sort of thing that could go under a rear seat.
Anyway, a quick google search shows that Sundown SA series, ID Max, and JL W6 are all around 7" mounting depth or a little more. Alpine Type R is 5 7/8 which seems right on the edge of what may or may not fit for you. I'm going to have to pass on suggesting anything with less mounting depth or 6.5" because I just don't have enough experience with 6.5" and have yet to see a "shallow mount" woofer that really impressed me. Not saying they don't exist, just out of the realm I can speak with any experience. I will say that I'd probably lean towards finding a reputable 10" that would make mounting depth over multiple 6.5" woofers.
