Use the foam from an Associated B4/T4 battery spacer and don't use the stock bladder at all. Glue the foam into the shock cap with something solid but non permanent (shoe goo?) and fill the shock till the oil is concave on the top and put some oil in the shock cap too(the idea here is too have no air at all). The foam compresses under pressure and that provides volume compensation.
Another trick that works well is to get K2 shock caps. They don't go through bladders as often from what I'm told.