I finally decided to upgrade the internals of the StableBuffer to match the base implementation (using [var ?X] instead of [var X]. (thanks @tomijaga for your contribution).
This should make the task of extending StableBuffer to match the base buffer library implementation much easier/quicker.
Getting an error on this run on the 0.2.0 branch where we use the new let else symtax. I think moc 0.8.7 needs to be copied to the dfx cache? Any chance you could make that change and a pull request? Hopefully we get dfx 14 soon.