Try just filling them up to the 5 gallon mark. That should help with the weight. I keep mine filled all the way in the summer but cut it back to the 6 gal mark in the winter to avoid freeze damage.

My beef with the vent plug is that it will never seal as well as a screw vent and it provides an entry point for bacteria and dirt. I guess if the contents are rotated frequently enough this shouldn't be too much of a concern.