I don't think the outbox/gossip model is just what is written in NIP-65. That is a part of it. See nostr:naddr1qqykycekxd3nxdpcvgq3zamnwvaz7tmxd9shg6npvchxxmmdqgsrhuxx8l9ex335q7he0f09aej04zpazpl0ne2cgukyawd24mayt8grqsqqqa2803ksy8
I also don't think this kind of bounty is a good idea -- or any bounty. I'm pretty burned by bounties.
Although I did put a bunch of bounties for gossip model implementation in clients last year and they worked, but only for people who already wanted to implement them anyway.