Not enough time, but:
1. Get DD-WRT on it.
2. Do this:
SETUP
WAN: Disabled
Local IP: statisk
Gateway / Local DNS: IP of main router
Assign WAN to switch: yup
WIRELESS
Client Bridge (Routed)
Default GW: Manual
Gateway: IP of main router
SSID: As main router
WIRELESS SECURITY
Same as for main router
SECURITY
Firewall: off
-ish.
Sorry, not enough time.