One thing people keep asking for is a dust filter solution. I did like @poweredbyporkers and used one of the magnetic 360mm filters, but because there is extra spacing between the front fans, these are a bit too small and cut off some airflow at the ends.
A properly sized and colored magnetic filter for the front of the unit would be one idea. Something that matches the white and black choices, with and without the light blue graphics, and similarly for whatever new Ubiquity inspired design, although if that one is aluminum it probably can’t be magnetic. You might consider rounder corners to avoid the thumbscrews.
Speaking of thumbscrews, I’ve taken to securing my HL15s in the rack with a coulple of them;
It makes it easier to secure/unsecure the unit than phillips head rack screws if you aren’t using rack studs. One thing is the tops of the ones I could find aren’t as deep as the thumbscrews on the HL15. For swag you could offer a packet of thumbscrews similar to the ones used for securing the lid, a bit wider in diameter and maybe with a “45” on them or something, M6x16 worked for my rack, but I think some raclks are M5.
You’d want to match the black and white. Bonus points if they could be captive like the ones for the lid, but that may be hard with the U opening on the rack ears…