The method I used that has never slipped at all was to use a drop of cynoacrolyte (super glue) on the fold-over, then put a piece of shrink tubing over that. I’ve never had the slightest bit of belt slip, it’s still holding tight as can be a year and a bit later.
I tried melting a section of belt cuttoff, it takes the heat very well, you would have to get really carried away with a heat gun to affect it. Nice thing with shrink tubing is you can slip the tubing off to make adjustments and put it back on, holds great.
I just used a piece of electrical tape around the belts. In reality all you will really need is a little external force to hold the two belts together in the vertical direction. (Shrink tubing works too). The horizontal forces tugging on the belt are held together with a mechanical hold facilitated by the teeth in the belt.