The only thing i can think of that would be remotely suitable is something called a CG LOCK. A team of guys were using one when competing in the Toyota Sprint Series in the UK. It's claimed to be nearly as good as a race harness but without going to all of the effort of fitting one, especially when space is at a premium. It works by keeping the lap section of a standard belt locked tight and doesn't come loose, thus holding you tight in your seat whilst cornering hard. It's easy to fit and a lot of Roadster drivers who've encountered your problem swear by it. It was only available in the UK but chances are there is a US supplier as its been on the market now since February 2009. The company can be contacted on www.cg-lock.co.uk
. From what i remember it retailed at around £55 to £60 and what with the rate of £ versus the $ you could get get yourself a great deal. It seems that its the safest and easiest fix for the limited space of the Roadsters.
These can be bought from Amazon