I would prefer one of declare, def or decl, because create very much sounds like running a ctor - so essentially, the exact opposite of what the syntax achieves.
Other than that: LGTM!
IMHO the best way forward.

I updated to use decl/declare keyword. Since it's a keyword that doesn't exist in jinja, we can always pick another keyword later on (cc @Kijewski). Added the warning for let with empty values and it will allow us to have let blocks after the next release.