my guess is you could cut and cap some appropriately size pvc pipe for your container/mold...pbby be a PITA, w/ substantial room for error, but could be done. as for the material, i have in the past used a compound to mold connections for underwater cables...we used these cables in the marine environment, so the material had to be durable. pretty certain it was 3M compound, black when mixed, came in a two part bag. will see if i can dig up some more info for you.
on a similar/another note: while i can't wait to get rid of my 'steps', i do make use of them when securing loads on the rack...anybody have an idea of which sliders might serve double duty? looked as if the shrockworks might offer a little more to stand on...any thoughts from those of you who might have seen both?