I have an assortment of old Delco 6x9's and 5.25's from old cars.  I just assumed everyone had a closet full of junk...

This old, cheap stuff that is not so good for other box styles seems to be a great fit for OB.

There is even a local surplus store that sells the OEM Delco stuff.  The 5.25" sells brand new for about $4.

You are going to want some big drivers if you want to get low with OB though.

I'm currently planning an OB bass augmenter manifold with B200's as the main driver.  They'll end up being quite compact floor standers with the woofer section 8-9" wide and 25" tall, which will contain 8 6"x9"s in each speaker.  They will be the equivalent of a pair of 15" woofer, but in a compact depth and drastically lower price.   Something very similar to my Flintstone cabs, but slightly taller.

Exactly, you just can't see the drivers through the front pathways because I put a piece of expanded metal over the opening from the back and those driver frames and entire chamber are painted black.  The work behind that is invisible but I was very worried about vibration noise from the grill and went to significant lengths to prevent it.  The new one will move a lot more air and will require a larger grill approach.  I'll also have to equally air mass load the back side too.

Thanks Dawg,

Those are about 2 years old and were one of my initial forays into OB.  I didn't plan for enough surface area on the augmenters and they sound really good, but only up to moderate levels.  The reason I want to try the 6x9's is that I can pack them into a more compact depth.  That way any depth in excess of 10" is 2 for one value and makes it a hybrid U-baffle.  That way I can keep it really compact, plus less pathway depth means I can cross as a higher frequency making main driver options more flexible.  My goal is to see how compact I can go with a high performance without cheating and boxing up the bass.  Whether I ever go to market with it is irrelevant to me.  I'm just having fun trying to do what no one else has, create a domestic size OB with real bass that can be driven by a single amp.