Should work out without too much trouble, as the expansion of the trapezioid is in the same direction as the horn expansion from throat to mouth. Bit more of a pain in the a$s to build though. Is this to match up with trap mid tops or to spread the dispersion more evenly?
Almost looks like you could load the driver either direction on the baffle on that last one. If you made it a rear loaded kick, it would reduce the front chamber, increasing the sealed chamber and might play a little wider range both up and down while still having pretty much the same length of horn.
Make a trapezoid version of my Omega Horn, and you will have a Funktion One RS2 lookalike.
I did some designs on paper but scrapped the idea because I didn't have much use for a trapezoid version.
I would suggest designing a new BPH instead of trying to get the HD15 in to a trapezoid enclosure.
Also consider a trapezoid W-bin maybe? There have some discussions on trapezoid kick bins on speakerplans before.
Mykey's C-4T is a nice example of the "horizontal expanding" arrangement.