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:
Premium polycarbonate aquarium lids & fish tank lids built to fit your tank. Stop jumping fish and reduce evaporation. Shop aquarium tops from Top Lids today.
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.
CNC-machined polycarbonate aquarium lids built to fit Red Sea, Waterbox, IM, and 300+ more tanks. Stop losing fish. Free shipping. Made in the USA.
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).
I have been in the saltwater industry since 1996 and have been working with wood and plastics since 2004. Every product I sell has been fabricated by myself, right here in Arizona! I specialize in custom products that nobody else offers. Don’t trust just anyone! Plastic and wood fabrication is...
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.
At Kraken Reef, our primary goal is to deliver top-tier premium rimless aquarium lids and accessories tailored to suit your specific tank requirements. Our lids come pre-designed and are available for various production model rimless aquariums.
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.