Hi Tony
Great how to substitute for a hard part to find. The booster boots are almost always shot.
Another suitable replacement that I have used following a similar process is off-road shock absorber boots. Which are easy to find seem to last well. The OD of the bellows on the ones I found allowed the retaining ring to slipped over and the mounting holes punched.
Here are a couple of photos.
You will note that the boot has more rings than the original
Below are pictures of the shock boot off.
Cheers Phil