29.4 down to 26.8 seatpost shim - does it exist?

I think you'll struggle. Could you try a 27.2 to 29.8 and sand a little out of the slot? It takes up the same 2.6mm difference and is only 0.4mm bigger so should conform when all clamped up. Especially if you use a thermoplastic USE one.
 
I use a 27.0 post in a 29.6 frame. I tried a couple 27.2-XX options. Works fine on and off road for many years now.
 
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