I always thought this is a neat setup. Even at one time fancied building me a set up like this. I still think it's a neat setup, but I have come to believe it is an aberration.
The first recollection I have of this set up is this picture from Round shot and Rammers
Notice it's a 10 inch seacoast mortar tube not a coehorn pattern. The drawing is supposed to be from a photo, but I have never seen the photo.
Here photo of the real thing
And a line drawing from AOP.
Later I saw this on Cannon-mania website.
Since then I don't know how many of these we have had posted here. But quite a few.
I guess what I am trying to say this isn't the right base for you little mortar. Not only doesn't it match, it's passe
I do like the one George posted. But be patient for a few days let the others dig one out of their books.