Need ideas on how to seal a vent NON permenantly

So I have a forced air intake vent in my 2x4 closet. I just recently bought a hepa filter to fit into it, but there is a small gap where unfiltered air passes through.

I have wood floors and do not want to damage them sealing up the gap, any suggestions? my first thought was duct tape, but I am pretty sure that will mess up the wood floor

Any suggestions on something i can use, and remove later without damaging my wood floors?

almost any kind of tape would work. electrical tape is not as tacky and can seal anything duct can and like duct tape comes in a variety of colors.

theres a form of adhesive tape called flash tape sticks like shit to a blanket but will not leave any residue on the surface but it retails at £25 for a 25 mtr roll.

painters tape is one option, though it actually does leave somewhat of a residue if left on after an extended period of time. A better option would be plastic protective removable film. It basically looks and acts like tape, but removable adhesive. Used on glass and plastic parts to prevent scratching during shipping. The same type of thing you can find in a hardware store is the plastic film you put over carpet or hardwood floors to protect it during remodeling.

they have draft guards for doors that might work?
http://www.homedepot.com/h_d1/N-5yc1v/R-202488279/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=draft&storeId=10051