The Glo and Glo HD are very similar in size (157mm x 114mm x 10mm for the Glo, and 157mm x 115mm x 9.5mm for the Glo HD), I have both and covers for both. When I got my first Glo HD, I just popped it into my old Glo cover, it was a bit more snug but it fit because of being a bit thinner. After maybe 6 months though, the stress on the plastic sides was enough to eventually crack in a corner, but by then a good assortment of covers for the Glo HD were out.
I swapped my Glo into the Glo HD case and I couldn't manage to get it in, I think it's more to do with the bezel shape because of the extra .5mm thickness. I probably could have forced it, but I didn't want to risk cracking the plastic corners of the case again, since I use it for my current reader.
This is the type of case I'm using, the plastic shell that holds the reader in place is very rigid.
If you can manage to find a cover with a bit more softer plastic, it should fit, but I'd be sure you buy it from a store you can make a return at.