I have been using a product called ShapeLock for some time now. It is a malleable, plastic based construction material that has infinite uses.
It comes in a bag of round, white beads. The beads are placed in hot water for a short amount of time until they become translucent. Once translucent, the Shape Lock becomes similar to a clear putty. I find it incredibly easy to make jigs for various firearm components out of this material.
One example I can think of that I have used over and over again is a front sight base jig. Brownells sells a FSB jig, but if you read the reviews many people comment how it doesn’t work when they have free float rail X, Y, or Z installed. Solution? Shape Lock.
If you are unhappy with the outcome of the mold, the shape can be recycled into a new shape simply by placing it in hot water again. This lets you recycle the shape lock for years.
The completed jig will be a perfect fit for your project and if you are wrenching on your AR at home, this product will help tremendously. I have used it for a jig to hold the rear sight assembly while I install the windage roll pin as well as well as numerous other small projects.