What modifications did you do for the fit.
The Fender G&G case is VERY well built, and fits the JMJ Mustang like a glove around the body.
HOWEVER, the internal braces are located in different locations from where they are in the vintage cases.
The vintage cases have the right brace located about six inches to the right, sitting very close to the upper strap button.
This prevents the bass from sliding to the left when the case is positioned with the left side down.
The G&G case has room for the bass to slide about an inch to the left, allowing the G string tuner to lightly contact the padding at the left end of the case.
Also, the left brace is located so that the neck strap crosses over the nut. The vintage strap locates the strap at the first fret.
I am OCD enough that I decided to move the braces to the "vintage" locations.
Vintage case:
Stock Fender G&G:
I moved the braces to the vintage locations:
Moving the braces was NOT fun. It's not hard in principle, but is very "fiddly". I really wouldn't recommend it unless you just HAVE to try!
I had to pry the braces out, remove the mounting nails, and then fill and hide the nail holes in the outer case.
Then I relocated the braces and drilled new nail holes in the proper locations. The only aggravating complication was that the new nail holes near the handle just HAD to be located RIGHT where the Fender nameplate is, requiring that the plate be temporarily removed.
Anyway, the case now fits exactly like the vintage ones!