When you build your Little Joe II, you'll want to make sure the bottom motor-mount ring is recessed about 4.5mm from the bottom of the body tube. This lets the nozzle bayonet plate sit just about flush with the bottom of the body tube.
Finish painting your nozzles before attaching to the bayonet plate. Much easier.
Unfortunately, I noticed pretty late that I made the bayonet mount a left-hand thread - means left-turn-to-attach, backward from what most of us would expect. Oops. Print a mirror-image of the part if you're a stickler.
The nozzle plate is flattened on two sides so I could print with my relatively small 130x100mm bed area.
If you're a 3D printing noob, getting a friend to print for you, using a shared printer, etc., these are pretty trouble-free and quick prints, if your printer's basic setup is good. Shouldn't be hard to kick out a good print of any of these parts.
I sliced in Cura without problem, used support for the bayonet clamps, printed with white PLA, and adhered the nozzles with Cyanoacrylate Adhesive (CA - Superglue).
The bulkhead is attached to the inside of the capsule with CA - time will tell if this holds well enough with a given amount of weight. I sanded the slots out a little to make sure I had the best possible fit.
The nose-plug is a simple twist-lock. I'll secure it from turning in-flight with a piece of tape. May require a tiny bit of sanding to make sure the fit is easy. Check this out before you CA the bulkhead in place.
Use your smallest nozzle and a slow print speed to get a nice result on the RCS motor. Print with support everywhere - and probably on a raft: the back is a little concave to sit down properly on a 4-in tube.