While cutting out some new slipper soles I made these out of a few leather scraps for my holdfasts.
Basically traced the shape and sewed all but the last two or three stitches then slid them on and put those last couple of stitches in place to hold them on.
I'm really happy with this mod, while I've had and liked the Gramercy holdfasts for a few years, this really drops the slippage to essentially zero.
A bit before that I also faced the front face of my vise with the same leather, also definitely worth it.
You can also see in that picture a couple of my square topped bench dogs which are also super handy. I use them a lot as a planning stop, or clamping on the surface of the bench off of the vise for carving. Pretty quick to knock out on the lathe.