I'm using an Edgestar/Koldfront 28 which has a smaller internal volume than the Newair 280, but used the HCM calculator and guidelines and ordered twice as much as it recommended, but in smaller bags so I could spread them out. I use a temperature controller to keep the temp between 65-70, and have a set of drawers/shelves from Forrest and about half the humidor is filled up with boxes (some of them empty). I also plugged the drain.
In my experience, the biggest thing is getting as much properly seasoned cedar in there as you can to help stabilize the humidity. The empty cigar boxes help by both taking up empty space and being made of cedar. I had issues getting humidity stable until I got all that cedar in there and as much empty space taken up as possible. Until the humidity got stable, I used Boveda packs along with the HCM beads (both 65%) but once I was holding right at 65% and recovering quickly, I removed the Bovedas and it's been solid ever since with just the HCM beads.