I also have my own implementation of packfiles [1], which I implemented as part of a toy Git implementation in Haskell [2]. I personally found the packfile format underdocumented, e.g. the official reference for the packfile generation strategy is an IRC transcript [3].
1: https://github.com/vaibhavsagar/duffer/blob/bad9c6c6cf09f717...
2: https://vaibhavsagar.com/blog/2017/08/13/i-haskell-a-git/
3: https://git-scm.com/docs/pack-heuristics/