[dsraven]

if fabbing up a spacer or filler to fit into the master's bore to make up the difference I would measure the required length then have one made from solid stock to fit the bore snug. bullet nosed on the inner end and with a depression to center the booster push rod on the outboard end. not a hollow affair that may have a chance of the booster push rod forcing itself through the end and into the hollow area, or wearing the end of the push rod to fit into the hollow bore.
just me though, overthinking things.it is a safety item though.
does the booster push rod adjust at all?
check the stroke on the master while it is accessible and then make sure your booster stroke will go at least that far when the pedal is pushed to the floor. this will ensure a full stroke with a full pedal travel. I know, disc brakes need less fluid for an application but if you have ever had a brake problem, like a blown brake flex line that uses a lot of fluid per stroke until the master is empty on that circuit, you will know what I mean. these things don't usually happen in the driveway.
good luck, keep the pics coming with what your fix looks like. great job troubleshooting.