One that would hold the universe together...

I made one back in the day. I mounted turntable hardware onto a round piece of wood. Then I took small lengths of wood, cut precisely, and glued them onto the table to make a frame for the galaxy to sit in. There was enough room around the galaxy on the turntable for all the common play area stuff (Strategy Cards, objectives, etc). It allowed players to rotate the board to reach everything comfortably. One of my friends loved it so much he paid me a hefty fee to buy it from me. I intended to make another one but never got around to it.

It would be nice if they made a puzzle-piece style frame to hold the galaxy together, though. Before my turntable, we would take a huge piece of glass and lay it on top of the map once we had completed galaxy building. The glass would hold the map in place so we would put all the game tokens on top of it without fear of moving stuff around. We made a big deal out of the act of putting on the glass (we called it "the laying of the glass") and would chant like it was a religious ritual. Maybe you had to be there, but it was always pretty funny.

They also have those pads that are used in RVs to prevent things slipping around. They're kind of bubbly and have good grip without being sticky. A puzzle piece boarder would be good, so long as it actually fits. The one for Catan is terrible.