Search on "SBIR" - Speaker Boundary Interference Response.
I believe the Realtraps website has a good article
The easiest and best treatment is not to put one speaker into a corner
, as symmetry is important
. Its much easier and much more effective to avoid creating the problem
than to try to remediate it.
And I am sorry you have emotional issues with Fiberglas. But you will find most of us have more significant issues with recommending foam! Do a few searches on Corner Bass Traps (you may
find 1 or 2 hits if lucky) and Panel Absorbers (maybe
2 or 3 hits on that as well).
And if that fails, try reading
the threads in this portion.
If my memory has not totally failed me, I seem to recall one or two posts addressing topics such as SBIR and the use of absorption, as well as various types of absorption - be it semi-rigid or loose fill Fiberglass, Rockwool, Cotton batting, and any number of other materials - and even
the plans for making panels or corner traps.
Dare we ask: Have you read anything in this section of the forum?