Hi,
The cage we made the bar cover for was actually a Gabber Rex, which only has bars on the roof. Imac Fantasies have bars all the way around, so I'm not sure if it would work as the plastic needs to be on the inside of the cage or else the hamster could still get it's nose through the bars. I think you could probably manage to do one flat side and tie to the bars with cable ties, but you would have to be especially careful to make sure all the edges were smooth so the hamster couldn't hurt itself. I'm not really sure what you could do at the corners though.
The only other think I'd be concerned about would be making sure that the hamster could get enough air so you may have to drill air holes, but these would probably encourage the hamster to chew at them. So sorry I'm not sure if this is much help!
You could try providing other things to chew at such as nibble sticks and chews, or you could try weaving strips of cardboard through the bars. I've heard that this sometimes helps to stop them chewing. If all else fails then it would probably be best to save up for a tank-type cage which your hamster can't chew at.