A basic heat box? That should work although two air filters will cut the CFM of the fan dramatically. The other thing is the flexwatt it is a radiant heat source it will not directly heat air but the surrounding material that in turn heats air. Like trying to heat your house with a barbecue outside by heating rocks. It can be done bust is very inefficient. I'd look to a convection source like a CHE or perhaps a goldenrod they are both convection sources. Just remember all convection sources drop RH by raising ambient air temps but that is the trade off.

http://www.goldenroddehumidifiers.co...ifications.htm