I make my oil pans out of a roll of 18" wide galvanized steel. I bend a 3/4 inch lip on all 4 sides. You have to have the pan closed in so that the bees cannot get to it. Put a #8 wire cloth on the bottom of the hive so that when the bees harass the SHBs, they drop to the floor and go through the #8 wire. I build the bottoms of my hives with the wire for the floor and add a board around the bottom edge so that the pans rest on it and I staple window screen over it so that the bees cannot get to the pan. I staple a board onto the rear side of the pan that is cut to the size of the opening in the back side of the hive. I put a little handle on that board so that I can pull the board/pan out easily for cleaning.
I use vegetable oil because it is low smell and the cheapest one I can get. If I could get mineral oil at the same price, I would use that.