Neil
If you use an outside mold, just put the whole thing on the dish. If for some reason you don't want to that, put the rim in the mold and insert wedges along the bottom edge of the rim every couple of inches to support it against a flat backboard while you glue on the top, then flip it over and do the same with the back. No dish needed. The shot below shows the wedges under the bottom edge of the rim to make the top edge of the rim extend an even amount above the mold.
https://www.flickr.com/photos/ruby16...7649776959267/
Click left and right, text below. I find it impossible to attach a photo to this format.
Merry Christmas
Ed