4th/5th axis head "quick" construction

I have built a 3 axis cnc that i want to change to a 5 axis. There are different "4th axis" modules on ebay and i was thinking of buying 2 of them and cast/mill some parts to make a bc head.




The chuck and a nut i will add on the backside of the C-module will clamp to a shaft from Z-axis.
Then i will create a thick L-profile aluminum part that i attach to the C-module base. Now i have a flat surface on the back side (opposite chuck) of the C-module.
On that flat surface i mount the base of the B-module.
I then mount the spindle to B-module chuck with some simple adapter part i will cast.

Hope you understand what i am trying to do.
Does this sound reasonable for a bc head?

I will mill plugs and molds for fiberglass. Probably not in aluminium so the load should not be that great.