How much clearance are you needing to avoid heelstrike? Which rear rack do you have now?
One of the longest racks I've seen is the Tortec Expedition. It sits pretty high which gives you plenty of tire clearance for 27" tires w/ fenders. Made of 10mm aluminum tubing. Only available in the U.K. I ordered two from Evans Cycles which delivers worldwide. Currently it's about USD $52 with shipping to the U.S. If you need dimensions to see if it will work for you, let me know.