I know this is an old thread, but I think I know how to do this!
I tried to draw it, kind of hard... But yeah. This is so hard to explain, but if you make a tube of silicone, and the ends of the tube are folded inwards, then you attach one ending of the tube unfolded onto the shaft and attach the other end when it's folded onto the top of the shaft. Wich means that when you pull the skintube backwards that ending unfolds and the other ending gets folded. Almost like it slides, but it actually just folds and unfolds!
The red drawing is the shaft, the green one is the skintube and the yellow is where it is attached to the shaft.
The blue is attached to the body to cover the fold on the root of the shaft!
The top one is what it would look like when it's pulled back and the bottom is what it looks like when it's pulled forward.
The green painting to the side shows the skin tube on the inside, if you had cut it in half.
You should make the folds just as long as the glans, because the folds is how much you will be able to retract the foreskin. Then you can just create a silicone frenulum and attach where you want it to be!
I think you should make it tight because otherwise the folds will be clumsy!