Unless the seats are "loose" They should fit tight and the latch shouldn't be necessary. That being said, if something is in the trunk and you slam on the brakes without the back seats being latched, don't complain when you get knocked out. Have you looked into swapping your back seats for some from another vehicle? Might be easier than all the fab work.

Its funny that you call it a design flaw, at one point it was seen as a design advantage, much like the 90* doors.