Gloss clear coat for flat spray paint?

I recently spray painted an item with a Rustoleum oil-based enamel flat spray paint. It is the color I wanted, but the sheen (flat) is not. The color is not available in a gloss finish. I tried spray painting it with the Rustoleum clear gloss enamel, but after two sprays, it still seems dull.

Is there a clear high gloss enamel paint I can try? Or, can I try a water-based poly (so that it does not yellow)?

At this point, I need something I can brush on as the item is now in a location that I cannot move to spray. Thanks for your help!