Having researched a lot about subs before I built mine, I found that acoustic stuffing in a subwoofer is not as critical as you might think. The cabinet may have been left empty by design of REL. Adding acoustic stuffing can change/reduce the volume inside the cabinet and affect the subwoofer frequency response (if you add too much).
If you want to improve things on your subwoofer (and you’re not afraid to get your hands dirty), I would apply as Dynamat Xtreme to as much of the inner walls as possible. In a subwoofer like this, cabinet resonance is more critical than internal standing waves. You can see that it doesn’t really have much internal bracing to resist flex/vibration, so reducing the resonance of the cabinet walls will be an improvement.
If you really want to add stuffing, then first apply the Dynamat. Then use spray glue to coat the inner walls one at a time. Then immediate apply a 1-2" layer of acoustic stuffing to the wall you are working on. A good product is Acousta-Stuf Polyfill from Parts Express. It is not fiberglass, so you can handle it with bare hands. It is easy to pull apart for any thickness. The spray glue is available from any hardware store or Home Depot. I have used products such as 3M Super 77 or Loktite spray adhesive.
I do not have any acoustic stuffing in my vented subs and they sound great. Instead, I built very extensive bracing internally, so that there is not one section of wall greater than 8" that is not braced.