Putting together something similar myself right now, using an open top 116 litre (i think) poly drum with a water/air tight seal on the lid that i got from work.
Personally i dont like PVC, shatters fairly easy, threaded caps can get damaged fast, no gurantee its going to be water/air tight, costs and weighs too damn much for what it is.
For the bottom i'd throw down a layer of good quality dessicant with a layer of absorbant cloth on top. Rubber would work for the sides but may add more weight than you want. Assuming you can get a good seal on it and have dessicant i dont see a problem with using foam.
If i was you i'd start looking around for a drum supply company and see what they've got in the way of poly drums, may end up being cheaper than PVC.