Stack quarters, dude.
Make your own spacer with quarters, down the receiver extension. Then put in the spring, and buffer. It's a common thing when building a 9mm AR, to keep the bolt carrier from over-travelling, which in turn breaks bolt catches. On a 9mm AR, using a carbine-length buffer, it take $1.75 to get the job done. 7 quarters, which equal about 0.470". If you need 3/8" of space taken up (0.375"),then stack up $1.25 (0.334"), or $1.50 (0.400"), and check the bolt carrier travel and the bolt catch operation.