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Re carrying/storing a spare belt - Gates say.....https://youtu.be/VgM-qeHGK_4?si=juJL4eavETswXT8s
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I was surprised to see that Gates recommends that.  I do not recall where, but somewhere several years ago I saw that it was clearly stated that if you did that, it would damage the belt.  I might have seen that on the Thorn website somewhere.

But if Gates says it is ok, then it clearly must be ok.

There used to be a discount bike parts company in USA (Nashbar) that would pack their wire bead tires that way when you bought a tire from them, shipping boxes were much smaller that way.  Before I put a tire on a rim, I would have to unkink the wire bead in a few spots.

A decade ago on my Iceland trip, I met this couple there, see photo.  They were a mixed marriage, one had belt and the other chain drive on their Rohloff bikes.  They said they carried a spare belt because nobody in all of Iceland sold them.

It shouldn't require anything, I have two Thorns with this style of BB and had a previous bike with something similar, they've never needed anything other than the bolts pinched tight.  Not even super tight, Thorn's first version of this style of EBB had four bolts, I assume they dropped that because it wasn't necessary. I put a bit of copperslip on mine, to avoid galvanic corrosion, but it wouldn't stop it slipping, if anything it would increase the likelihood.
Sorry that's not very helpful. It's such a simple mechanical design, if the parts are within tolerance I can't think why it wouldn't hold.  These are used on tandems, so it isn't likely you're just to powerful for it! Have you had the bike long?  Has it always done this? Do the lugs on the split meet when tightened?  I had that on a seatpost clamp, which is basically the same design. It's maybe not impossible that the insert is slightly undersized, if you have some accurate callipers, give it a measure. If you don't find a simple solution, maybe give Thorn a ring, it's possible they've come across it before. 
Good luck.

Apologies just saw this. I imagine you have a frame now. I have almost finished building this up into a finished bike...

Tristan Ridley who is a long distance back packer has moved back to chain from belt.  He has a YT video explaining why….

Available here...
https://www.youtube.com/watch?v=nx5nN3kyx5k

Best, Dan.

For Sale in BH7
Black Thorn Sherpa 511L 50cm
9 speed good condition £650
See EBay or msg me on here
Cheers
Robert in Bournemouth