Well, something about the base was just 'not right' and I couldn't put my finger on it. Then I studied all of the images again and it finally hit me, the bases are not square.
I designed my base square (and a bit oversized it turns out), and it just never looked right. Reducing the base dimensions to 4½" x 5" at the bottom puts the whole thing better into proprtion with the images of the eprouvettes made by BNA ad SBR.
My eprouvette was never intended to be a dead-on copy, rather a simplified interpretation, but certain proportions are important. If the base looks wrong, the whole mortar looks wrong.
By my calculations, my powder chamber will hold about 200 grains of powder. Maybe that's too much, but that's about par with most of the Coehorn plans that I've seen posted here. I'd rather have reserve capacity than a too-small chamber.
By comparison, the original DuPunt plan (full-size) has a powder chamber that holds about 320 grains and it throws a 24 pound ball. Of course, being a testing instrument keeping the range short was to their advantage. The plan is sorely lacking in details, though, and the few dimensions it gives are barely readable.
A firing fixture like the one above will be a must-have accessory, I'm sure. That's why I incorporated the beveled base of the original into my plans. I probably will succumb to the temptation to fire heavier balls.