I have the same question. I've got a RSR XL300 that came with the Red Sea cover. It's pretty chintzy. Lead times seem long for all covers and some companies seem to be a challenge to deal with. Here are a few I've found:
https://toplids.com/I've read some negative reviews of these guys taking forever and being difficult to deal with. Seems like a good product if you can get one. 3-5 week lead time.
https://www.clearviewlids.com/Apparently this is the same as Artfully Acrylic? Seems to be a good value compared to the others. Their Pisces EXO design is modular which is a big plus in case your equipment changes and you need cutouts in a different spot or a different size. 20 day lead time (not sure if that's business or calendar).
https://octoaquatics.com/Guy seems to be pretty proud to be "the first" to make these and thinks that anyone else who makes these is just ripping him off. That doesn't inspire me to order from him though. His FAQ is pretty bristly and defensive and prices are nowhere to be seen for the lids. 6-8 week lead time.
https://www.krakenreef.com/Just read about these guys on this thread. Nearly everything is an option which really jacks up the price, even something as common as a cord cutout. The feed cup is nice though. 2-4 week lead time.
I am leaning towards Clearview or Kraken.