It's a piece that is reused when changing shifters .... but it would be so very very easy to make one from a few simple measurements, a vice, a piece of steel about 13/16" wide and say .... 1/8" thick and less than 8" long, add 4 bends, two holes, done.

Maybe someone will make a few measurements for you .... or you could simply measure the block of rubber there, make the U with mounting ears.
On width of the U part, make inside same as width of factory rubber, about 1-7/16" inside the U should do nicely (don't worry if the bends are some rounded and not sharp bends, the rubber is rounded some too at corners) ....
.... and on depth make it just deep enough to snug the factory rubber against the floor, about 1-1/2" inside measure.
Add a "wing" on each side about 1-1/2" or so each ....
.... then simply drill two 1/4" holes same distance apart as your mustang's two studs.

Clean and add paint.

I didn't crawl under the car, I just measured the red poly rear bushing that Hurst sent that fit my '08's bracket so nicely .... but I reused the OEM rubbers as less noise transmission to floor.