Like MSTroh i do fuel proof my engine box inside and out but i use a polyurethane spray. I mount every thing to the point that its ready to fly cg the plane then when the cg is set i remove every thing from the engine box spray it inside and out (Making sure i get all the holes i drilled in the box) let dry for how ever long it needs then spray it once more then let that dry over night then remount the goods while blue locktiting every thing that needs it then its ready to fly

Edit; if the carb is open to the inside of the plane make sure you fuel proof the inside every where fuel can drop or make a plate to cover the inside of the fuse. This is because the fuel will drip from the carb.