So the outer 15 in tube was capped on the top? I'm trying to visualize this thing: 12in. driver facing up at the floor attached to
a tube (what length I wonder?) Then a 15 in. tube inserted over it
with little legs or something keeping it off the floor. The top would have to be capped then ennit?

Ceramic tiles would work well on a base too I think.

I've made the switch on these tubes putting the driver on the
shorter lower front tube and the open end with an elbow at the
rear. On the other one I put an extra length of pipe on
the rear portion with no elbow like a smoke stack. Too early to
say if this is practical plus it is not cut to a specific length.
Overall the length of that one from throat of the terminus to driver is... omigod 9 ft 9 in... just shy of the 10 ft 3/4 wave length.
If I put the elbow back on I'd have it all.

Soundwise, I have no test equipment so not sure about that last one up there. I spoze I should figure out what I have here by calculation but the 8.5 length is the closest thing to 'fitting a room' that I've come up with and sounding good to boot. I'm starting to get curious about how a different driver would sound.