I assume the sign in the picture is not small. An eight or ten foot diameter cabinet on top?
Plastic trim cap is not a suitable retainer for a face like this.
Use .080" aluminum to fabricate a retainer. I would cut the face of the retainer with a sabre saw by hand. Then weld the return to the retainer's face as I bend the return flush to the edge of the face. It's been many years since I've fabricated a curved retainer like this, but I think that welds every inch or so is about right, butting the return piece against the back of the face piece. It's time-consuming, especially for me since I came up as a painter, not a fabricator. When I was asked to fabricate in the shop it was because everyone else was way busy and they were fairly sure I wouldn't screw it up.
On a round cabinet you can make the retainer in two pieces, a top and a bottom. Leave the bottom piece attached permanently to the cabinet. The top half can be removable so the sign face can be lifted out for servicing.
If you are having a capable fabricator build the cabinet, they should also be able to fab the retainer for you if you don't want to try making it yourself. .080" aluminum is easy to burn through if you're not used to welding it.
The illustration above is showing the back side of the retainer, of course.
I think I could have fabricated one of these in the amount of time it took me to figure out how to do the drawing in Signlab.
Brad in Kansas City