With an 18" driver and inner tube, the tube area would be 254.47 in^2. The next tube size up would ideally be 25.46" for equal area (assuming zero wall thickness), and the one after that 31.18", which is about the width of a doorway and would represent the upper limit if mobility is an issue. Still, that gives three multiples of whatever length (height?) the space could accommodate, so a quarter-wave TL that gives great extension should be possible, provided this idea is sound - I don't know if the waveguide has to be a contiguous area or if it would still work in an annular space.

Thoughts?