Putting a fan in the existing ventilation opening is probably the best solution. You could make it temporary by using plug-in power connections.
Dont forget to allow for make-up air to replace that which you are exhausting. The half-inch gap under the door will work, but it will not be lightproof. A solution would be another door as JOVO suggested, with a light-proof grill that you can purchase from most photo stores. An even better solution would be to install a light-proof fan in the new door, pulling in air and helping the exhaust fan work.
You room size works out to about 427 cubic feet. I would allow at least 6 air changes per hour, which works out to about 50 CFM. But these fans are not very efficient, and they will not be in the ideal locations (over the chemicals), so I would use 100 CFM fans.