It's important to note that the higher the shrink ratio, the thicker and stiffer the sidewalls are. Also the higher the ratio, the tighter the heatshrink will clamp around the object.
Also when you buy heatshrink for this purpose, make sure to get heatshrink WITHOUT the built-in adhesive. Sometimes it can have an adhesive on the inside so it can't slide out of place and it can positively seal wire connections. That would be bad if it glued itself to your fork.
From my experiences with heatshrink, I would recommend putting a moderate coat of dielectric grease on the fork where you want the heatshrink to slide. The grease does three things:
1. It's grease. It lubricates.
2. It doesn't allow the heatshrink to constrict all the way down on the fork because the grease has a measurable thickness. The heatshrink would need a massive amount of force behind it to squish out that film of grease. This is the reason grease is used in bearings and bushings and yada yada yada...
3. When the heatshrink touches the grease, it stops shrinking. This is because the grease conforms to the inside surface of the heatshrink and wicks heat out of it VERY quickly, prohibiting it from shrinking any further.