I looked at GripGrab. The only MTB covers are the RaceAqua X, RaceThermo X, and Arctic X, at $57, $65, and $82 respectively. Limited reviews do state the RaceAqua has something of a slimmer fit-- same problem I have with my Pearl Izumi covers.

I already received my AeroTech covers, which are definitely big enough to fit over my CX shoes, and an absolute bargain at $29. A pricepoint I'm good with, as this is a product I bought hoping I need to wear them... like never. I mean, this is Socal. wear my bib tights less than 10 times a year. But I also hate sitting indoors just because it's a little sprinkly.

Sometimes I feel like having size 10 feet instead of 14 would solve a whole lot of problems for me. I'd probably fall down more often, though.