You could try to cut the slots out with a Deemmel and a thin cutoff disk. Then go at them with a proper sized screwdriver blade. How about a impact driver tool? The kind that has a screwdriver bit, and you hit tpo with a hammer?
A little heat may help, and of course your favorite snake oil. Have had good luck with propane torch and candle wax on stubborn threaded things.
My NH actually uses hex head bolts with no problems with hay snagging, etc.