For the cookie pan, I'd cut around the edge right in the center of the rolled part. This will make the lip less thick, but still keep a nice edge for mounting. Make a cutout in the case panel, insert the cookie sheet from the inside. For attaching it, I'd look at dropping some pop rivets through the cookie sheet rim, just at the very edge of the panel cutout. Put the head on the outside of the panel, and the rivet gun on the inside. This will attach it solidly, and the rivets can be hidden with a nice fillet of JB Weld on the outside.
Painting is a ton of fun. Masking can be a pain, but yes, the final reveal is always a treat. I usually do all my painting in reverse on the back of acrylic sheets, but I can also cheat a bit as I have a laser. So it either cuts the masks for me, or I can engrave off the paint to provide another color, or a clear area.
I have a hammer! I can put things together! I can knock things apart! I can alter my environment at will and make an incredible din all the while! -Calvin