Personally, I'd argue that the proposed question would be off-topic.
It's not off-topic for all of the typical reasons that requests for recommendations of off-site resources are off-topic. That is, you're not really going to attract spam, given that you've limited it to models of Kobo readers. If you were, say, asking about covers that match your current reader, that would get much spammier answers.
The question is still off-topic, though, because it's about accessories for particular physical devices used to read ebooks, not about ebooks per se. In other words, the question is basically, "What Kobo models are the same size as a Kobo Glo, so that a cover designed for Model X will work on my Kobo Glo?" That's pretty far from being about ebooks themselves. On top of that, the answers would rapidly become outdated as the Kobo model lineup changes.
All of that said, we've been seeing more and more questions about devices lately, and we've been pretty tolerant of hardware-related questions ever since our first Meta question. So, additional community input here would be helpful. My opinion is just that -- an opinion -- and the ultimate decisions about what is on-topic are up to the community as a whole.
P.S. Just spit-balling here, and I'm not super-familiar with Kobo products, but if I'm looking at the right info on Kobo's site, the Kobo Glo is 157 x 114 x 10 mm, and the Kobo Glo HD is 157 x 115 x 9.2 mm. That's only a 1 mm difference in width and 0.8 mm in thickness, which implies the HD case might well work on your older model, depending on exactly how the case attaches.