I would like to suggest that the opening at the top and bottom be a bit on the small side.
you mean they're too small right now or too big? and how do ya figger that, teach?
the picture isn't very good, but the openings are about 2" x 24" right now. I could cut them larger and I could block part of them to make them smaller. I'll have to make little doors for the openings anyway, to close the heater at night.
what I like about the design is that the air is relatively unrestricted and can move through the heater quickly.
should mean more air moving through and being heated, albeit to a lower temperature than if it were forced into a convoluted path. if I make the openings too small, that advantage will be lost. I also like that it's got such a low profile compared to the soda can heaters. the heater itself only extends 1.5" beyond the plane of the door.
anyhow, let me know what your thinking is about the size of the openings. probably be easier to change them now than after I hang the thing.