Hey there!
I’ve just opened the first PR against the stateless-ethereum-specs repo: https://github.com/ethereum/stateless-ethereum-specs/pull/1
It has a draft of a block witness specification, based on the block witnesses that are used in turbo-geth with a few minor tweaks.
A good thing about this format is that it works 
Basically, we’ve been running tests on Ethereum mainnet using almost the same witness format and were able to proof blocks for the whole history of the chain.
I’m very open to any feedback regarding the spec itself and the witness format too.