Lylo sez: ... I have hopped into my cube many times.... ???? How in the world do you fit into it? Does it get easier each time?

Sorry, couldn't resist..
Re tightening the cap.. US Plastics strongly suggests tightening and sell a wrench for the purpose. YMMV. I'd assume using an incorrect wrench would be a problem.
My question is.. after buying two 5g/19L cubes.. do I have to be concerned if I only put 2.5-3 gallons into that large a cube? I searched the USPlastics site and they don't sell a cube smaller than that that is good for high temps. Conflicted here.. Do I have to up my game and go for 5 g batches again? I don't really want to have to do that. I know I oxygenate my wort before pitching... so, I don't know if I have to worry about that added air space above my wort.