Before even reading this post, i suggest you take a look at this thread first and read through all the pages so you understand what the J-hop is and the many possibilities available.
http://www.airsoftsociety.com/forums/f10/j-hop-34376/
Unfortunately, the thread on airsoft society has seemed to dwindle off a bit, and i haven't found much more testing done on this. If your going to take a stab at this mod, please share your results with everyone here so we can learn from each others mistakes and find a good solution. Once we find something that works, this will be a much easier method to making R/ER-hops or any type of hop for that matter.