[Uptown Girl]

I had good luck using The Company Store. (online or catalog)
They manufacture a (limited design) selection of 'super sized' and 'oversized' comforters, all made in Wisconsin and also carry some name brand merchandise in addition to their own stuff.

I purchased a lightweight 'oversized king' (110"x118" wide) - 8" to 10" wider than most standard kings. It required buying a duvet/comforter cover ( which I don't normally like) but they carry ones that are made to fit these (again, a limited selection )- I chose Charisma brand, and am pleased with the quality.

The comforters are built with a loop on each corner and the covers tie off to the loop on the inside, so everything stays in place, mercifully.

My ensemble is conservative in look, but quite comfortable to the feel and has totally eliminated the need to wrestle covers from my hubby during the night, which was my primary motivation.
(I made it spiffier using a couple of cool throw pillows.)

It would be one solution to cover the deep mattress you have.

These comforters/covers are not inexpensive, but worth it to me. Hubby likes it too.

I have ordered from this company in the past- always had a good experience. Worth a look.