You want some seals that will work and not cost much, right?
Get some pipe insulation (black foam stuff from Home Depot or hardware store) that has a slit on one side to wrap around the pipe. Cost- about $5
Get some double sided golf grip tape from your local golf supply house (GolfSmith or Golf Galaxy.) Cost- about $2
Pre-fit the pipe insulation to the sides of the radiator. See where the insulation meets metal. Remove and place a length of the golf grip tape on the insulation. Peel off the paper backing on the grip tape and work the insulation back into place.
It won't go anywhere.
If you need to double up the insulation, slit it and use the golf grip tape to build it up to the size you need. Having some rebound to the insulation once its wedged in is good- will help seal the gaps.
Oh, and you will have a lot of grip tape left over. Good for all kinds of stick 'em jobs (loose carpet, etc.)