i got one of those awhile back so i made some flaps out of a heavy duty rubber door mat that cover my 37s and just bolted them to my stock flap bolt holes,
but if u dont have the stock mounting areas u can do something like a quick release flap

youll want something that can be taken off quickly for when u go wheeling. find some square stock maybe 4 inches long and weld that to the frame however far back u want the flap,then find some smaller stock that fits perfectly inside the piece u just welded to the frame,this piece make as long as your flap and bolt your flap to it,then where it slides into the piece welded to the frame drill a hole and put a quick release pin in that. problem solved.
if u search around u will find some threads on this with pictures to help explain.