Hardboard should work fine...since you'll have the flexwatt on a thermostat it won't get hot enough to cause problems. And even if something went wacky, it would take a lot of heat to affect the hardboard.

As for attaching it, just make sure it's cut to the dimensions of each shelf (under size is better than over), and drill some holes one on either side of a cross bar section to wire/ziptie the board to the shelf.

There is nothing wrong with using that shelf unit for a rack. I have a few of the 'garage storage' plastic units that hold tubs and tanks here and they work great!
Oh, and if you find you need to 'enclose' the rack by putting sides and/or a back on it, just use coroplast corrugated plastic or hardboard. Cheap, easy to install, and still looks good.