Yes, G motors are different lengths, ranging from Econojets at 3.875", Aerotech SU motors are 4.875", and Ellis Mtn SU's are longer (6.5"?). 29mm H motors are even longer, up to 10+".
You can glue a thrust ring for a motor block (cut a piece out of used motor casing is cheapest) inside the mmt tube, but that limits you to whatever point you put the block in. I used a block for my first MPR, set at 4.875" for a Aerotech SU, which is fine, but now I'd like to be able to use an H in it and can't.
What I use now are piano wire hooks, usually two. I got a 36" length of 0.55" piano wire from the local hobby shop (only $0.75) and cut it to about 6" or 7". Using pliers, bend a 1/4" length 90 degrees on the aft (nozzle) end of the wire (for motor retention.) Then...and this is the tricky part...bend a 3/8" length 90 degrees in the EXACT OPPOSITE DIRECTION on the forward end. (Sorry, I don't have pic to show you right now, think zigzag.) Next, cut a groove in one of your centering rings. The bent forward part fits in the groove and will both hold the piano wire hook in place and keep it from twisting loose. Generous epoxy helps. I usually use two hooks on my rockets for redundency.
You will still need to use masking tape as a thrust ring at the nozzle end of your motor to keep it from thrusting forward, but you don't have anything in the mmt tube to limit your motor length.
Kinda rough, but it works on my rockets.
As for a lathe, you gotta variable speed drill?? I've done nose cones up to 3" in diameter by drilling a hole in the center of a balsa block, gluing in an oak dowel (gotta be oak, pine or poplar will shread), trimming the corners, then putting the dowel in the chuck of the drill and turning away. A 60 grit sandpaper block or wood rasp does a nice job of shaping quickly, emory boards are best for fine work.