Nothing for safety. In fact, HDPE melts at 275*. The assumption is we're not going to have a fire. LOL
I have built a lot of stuff with wood - cabinets, boats, etc. and find the look and feel of starboard to be superior to plywood. Had I used the latter I would have used a high-grade hardwood veneered cabinet grade material with lots of plys - like 7 or 9 ply birch or maple. Just because it mills up nicely, you don't get voids and splits and the like. And my OCD would cause me to want to somehow seal it or paint it, even though my battery sits under the sofa in the RV.
So when I saw the Carson Van pack it really clicked with me - I loved the way it looked, and knew right away then that I wanted to use starboard instead of plywood.
Some of the other compression jigs use quarter inch plate aluminum end plates and they look really nice too. I just don't like the idea of all that conductive metal around a battery. Same with the welded steel setups - and then there's the battery-fire thread with the bus. My biggest fear with this technology is the amount of energy these cells store - let out uncontrolled and it's a recipe for disaster.
And then there's the whole 'do I compress or not' discussion on prismatic cells. Some manufactures rate their number of cycles higher when the cells are compressed - not allowing them to puff. And then there's the distortion of the terminals being pulled by rigid connecting bus bars should the pack puff and expand. So you see most packs with some kind of compression setup.
Anyway, that's my 2-cents, having just gone thru the same quandary.