Hi Kim,
What a neat look!
I suggest using a doming epoxy resin. We have several in our store here:
https://shop.resinobsession.com/collections/resin/doming
Wait until it starts to cure a bit to make sure it is thick enough. It looks like your surfaces are curved, so you want to be sure there is enough surface tension for the resin to stay on.
I don’t know that you need a brush so much as a toothpick and a steady hand. The resin should spread out where it needs to go. Go slowly — put on a bit of resin and let it spread out. Once you break the surface tension by the resin going over the side, you will have to wipe it off and start over.