Yes, they will rust even with coolant. Look at a set of original nipples the insides are always corroded and a bear to remove.There is a reason why water pump manufacturers galvanize their pump impellers .. Coolant passages in a block are much thicker than the metal the nipple is made out of ,not including the increased pressure through the small diameter nipple that the water passages dont experience..., A piece of sheet metal to turn the slot? Not too sure if that would work.,Reason why is ,The slot is .125 wide and only .150 deep,Since the steel nipples are pipe thread and tapered its very important to make sure the nipples are tight. A peice of sheet metal closest to that size would be 11 ga (around .119 thick) Since its only going down .150 into the slot ,I wouldn't think that you could get it tight enough before the sheet metal fatigues. This is why the aftermarket ones you buy from the Auto Parts Store have a hex on the end to be tightened .These are milled to fit the slot and are made from 316 Stainless steel.They are $40 plus $5 shipping USA. I only made a short supply of these. You can email me direct at jrivinius@sc.rr.com for a paypal invoice. Thank you Moparts