Perhaps I should clarify the drawing a bit or how it would be built. It's basically the same principle as a household 'radiator' (which are really convectors). I have changed my thought slightly by putting the mat on the viv and not the lid as shown in the picture. Makes far more sense.

1 - On the side of the viv, drill two rows of holes. One towards the top, one towards the bottom. Big enough for air to pass through. these could just as eaily be two 1" wide 'slots' covered in a snake-proof mesh.

2 - Nail/Screw/glue 1" square wooden dowel around the edge of the viv

3 - Cut piece of thick plywood in a square (in the case of my viv, 15x18) to for a lid. Again cut an air slot at the top and bottom. Onto the inside of this, cut and glue as many bits of 1" copper water tubing as will fit vertically between the holes/slots you made.

4 - Onto the side of the viv inside the square frame you made in step 2, attach your heat mat between the slots.

5 - Screw the lid down so the mat presses against the tubing.

So, the convector heats the air inside the tubing heated by the mat. This warm air rises and leaves via the top holes and thus draws in cooler air from the bottom. It therefore creates an airflow circulation inside the viv. The slots on the lid would be able to draw fresh air in from the bottom and expel overhumid air from the top; the apertures of the slots being controlled with tape or something for the best balance.

I guess it may be a little complex, but not more than 45mins work. I thought that it seemed to answer a few of the environmental control factors that need to be considered. Background heat, airflow, ventilation and humidity control and would keep an unsightly cable out of the tank.