Just put the jackstands on top of a floor jack, that should give you some extra height 
I made some risers out of some angle iron and 2x2 square tubing. Very sturdy, and there is no way the jackstand can slide off the top (captured by the angle iron). Makes my stands probably a foot taller. Would be hard for you to do if you don't have a welder though.
blocks of hardwood would be best, stacked like # as stated above.
