When I was testing the idea, I thought I could use the original cylinder with the key in it, like on the retrofit kits. I got to learn the hard way that a pin tumbler lock does not operate well under the axial load of a belt. Nuki and other systems that work with the original key are mounted on top of the lock and avoid this, but I wanted to keep the contraption as flat as possible, since this is a rim lock and it's already bulky.

I then partially copied the lock in CAD, on top of which I designed a mounting plate to fit two small bearings and support the short axle for the belt pulley and actuating cam.

Got this printed this with PETG (because it turned out that's what my brother had in the printer at the moment). Feels really solid, especially when assembled. The bearings press-fit nicely, lucky me.