Very nice, I like it! I'm doing the same, but I'm using a different style of chassis. I think I had looked at a similar reference to what you did. I considered using it, but I felt the metal chassis was inappropriate for Empire cannons. I went with a wood style that has a look like the current chassis. I actually built mine using the same technique. I laminated three sheets of 1mm thick plastic card to make mine. I didn't use scissors, but an X-acto knife, then after the glue was dry I trimed them and sanded them. Then I used some squadron white putty to fill in any remaining gaps and sanded them down again once it was dry. I used a dremel to cut the half moon shaped indents for the Mortar pegs to fit into. I'll have to get some pictures taken of my work soon
One slight criticism, does your crew have a platform they will be standing on to load that thing? The way you've built it the muzzle has to be at about shoulder or higher level and it would be a real pain in the butt to load it. The one in the picture looks like it is about 2.5 foot high. Your chassis looks to be about 4 foot tall.