You'll need to spread the rack stays a bit to use this option (not a big deal) but a male/female hex standoff should do the trick:
http://www.mcmaster.com/#cadinlnord/93655a451/=ei6fle
Use the standoff instead of the supplied bolt to secure the stay to the frame then attach the rack to the standoff using a proper length bolt. It'll be a slightly weaker arrangement than having the rack right against the frame but chances of that mattering are quite slim unless you do some seriously heavy loaded touring.