A while back I set up a friend with 2500 tanks and 4x5 with the intent on hand processing. We used HC110, and the extra volume of the tank was no issue because we used it one-shot and just diluted it more, but used the same amount of HC110 concentrate that we would have used in a smaller tank.

I'm not following you on the 'black plastic end cap.' Obviously without the light trap and center column the film will be spoiled when you take the rubber lid off to change the chemicals.