Why can't you accomplish glued in stability with a removable beam along these lines. The beam would slide into a permanently fastened receptor on the hull, it would be supported from all sides but could be bolted in so that the beam could be removed if needed.

If it was a composite beam the end might even have a very slight taper which would further enhance the fit.

Seems like a relatively simple solution of accomplishing both goals...that is the stiffest possible mounting while still allowing the beams to be removed.

Regards,
Seeker