The width of commercially available underquilts is designed to keep humans warm, not hammocks. The only reason to get a 90" wide UQ would be to keep the excess fabric warm, not you. Unless your diagonal lay turns into a width-wise lay, you should be fine.
My HG underquilt works fine with a 60" wide hammock and I suspect your UQ will also work fine with a 72" or 90" wide hammock.
I shudder to think what the weight of a 90" wide UQ would be, even if it were a 3/4 UQ.
Last edited by SilvrSurfr; 07-22-2012 at 15:10..