If you guys use PVC be sure to get the thin wall stuff. Its cheaper and lighter and will probably suit your needs of being waterproof.
I have a couple cz 455 rimfire barrels only stored in some 1" pvc with one end having a threaded pvc plug for access. Painted O.D. green no doubt.
I realize you guys are trying to stash a whole gun but thought I'd add the part about thin wall......so you will know it comes in differnt weights AKA pipe schedules.
I know more about PVC than I want to.