I'm not sure having lots of small mixers then joining them together is necessarily the best way to build an anonymization service.

One problem is the maximum amount you can mix is the min(all pool sizes), which is probably not that huge assuming reasonable fees.

Another problem is that - let's face it - running a mixer is going to be seriously risky for anyone who lives in the USA or Europe, so there aren't likely to be lots of people queuing up to do it. I would expect such a service would have to at minimum run as a Tor hidden service, as the legal risk involved with being what is effectively a dedicated money laundering service is large.

And yet another problem is that - as you point out in your paper - trust is essential for a mixer service. Building trust takes time and effort, also many people probably aren't that interested in mixing services which further slows down the trust building process.

So I think it's worth exploring not only a "mixnet" model of many co-operating coin mixers, but also a model in which a small number of sophisticated mixers with large coin pools and possibly special hardware can provide this service, whilst still providing the same kinds of privacy guarantees a large mixnet would provide.

This approach has a big advantage, namely that if big miners trusted the mixnet owner, they could set up deals to send large quantities of coins through the mixer 24/7, providing plenty of cover to make timing attacks harder and a big liquid pool.

For clients trusting the mixer on more than reputation alone, a secure platform that supports remote attestation would be ideal. Unfortunately there aren't many of them out there. Hal has practical experience with the IBM 4758 but sadly they aren't being sold anymore. Their replacement requires a contract with IBM to run arbitrary code, it seems.

The final weak point in zipslacks scheme is the mapping of local address -> destination address. This could be solved using an obfuscated table algorithm but it's probably not worth it as the delay between setting up the mix and actually sending the coins shouldn't be all that big.