I would prefer to see contolling one variable (length) and comparing effects of the other variable (chamber form). One might get away with deriving the effects of length emperically by changing the length and plotting the results for each. Generally not practical.
What could be done is to have ONE mortar with 'drop in' chambers - that would allow several forms to be tested AND control a lot of the variables.
DD and I both developed very similar styles of what he called the KISS style of mortar when we did the "we'll send you a few pieces of steel and a chunk of oak if you send us the postage" contest.
With those it is easy to change the chamber by making a replacement AND at the same time cut different lengths of tube. I still have a few pieces of the 1144SP that I could provide if anyone wanted to persue this project in golf-ball caliber.