J&L has a nice reflector that SHOULD please read that as SHOULD fit the fixture. I had one of those older ones and was really happy with it over a 90 gallon tank. Believe it or not that flat white finish inside the fixture is a fair reflector, not great but not bad either.
Here is a link to the page, I'd look at the 15 inch but two 24s may just fit.
http://www.jlaquatics.com/phpstore/s...category_ID=83
Doug