I was wrong about my tube. It is for 11X14 stuff. 2 of those taped end to end would hold an 8X20 with plenty of arch, I think. A couple of layers of black electrician's tape overlayed by some duct tape (we who have lived in Newport News, VA call it shipyard tape} should do the job.
The capacity for film of the working solution is quite good. It is the ascorbate that is used up first. I did some experiments to find the optimum initial ratio of ascorbic acid (as sodium ascorbate) to phenidone, holding phenidone and pH constant and varying ascorbate. The activity as measured by contrast index after a given development time increased with ascorbate concentration asymptotically to a ratio somewhere above 80:1 but rather slowly after 40:1. While the 80:1 ratio does not give much greater initial activity, it will have greater capacity both for film and for aerial oxidation. The ratio I use in PC-TEA is about 40:1.
You see that some conceptions of how to increase the activity of a PA or PQ developer are in error. If the initial ratio is less than 40:1, add more ascorbate. This means that if you are using ascorbic or isoascorbic acid, you need to add enough alkali to keep pH constant. If the ratio is at or above 40:1, add more of both phenidone and ascorbate in the ratio of 40:1. In other words, add a little more PC-TEA.