I have a dive buddy that took 4' of what I think is 18" diameter PVC and fitted a wood plug into the bottom. Cut outs in the side of the pvc allow it to air out and for water to drain. Usually holds two guns. Twin bungees hold it in place in a corner of the deck. I can try to get more info if you need it. The rig sits upright and the spearguns are inserted spear down. We have been using it for about 10 years.
---Bob
---Bob