Your use of "hacking" is confusing and possibly incorrect. AFAIK when "hack" is transitive it usually means violating, exploiting, or modifying an existing system, which it doesn't sound like you have yet.
"hacking together a startup" would probably be more clear, meaning that you're making it quickly out of technology without regard to elegance (for now).
"hacking on a startup" might be best, meaning that you're playfully modifying or exploiting (but not violating) the startup, but it still semi-implies that it's a pre-existing system. This also doesn't work with "in 3 weeks", so you'd have to change it to "for 3 weeks" or maybe say "hacking on a 3-week startup".
While I'm at it, the "leave kimono wide open" thing isn't really clear. It sounds like "some guys put together their startup too carelessly and got pwned."