I cut out all of the molding in the UGC except for the two straight pieces at the top and bottom; this seems to hold the NES carts just fine for my liking.
I use an Xacto knife very carefully. There's a couple of ways you can do it. The easiest way is to trace around the base of the molding you intend to remove with the cutting edge of the knife multiple times, pressing slightly harder each time until you basically cut the whole thing off like it was warm butter. It ain't the fastest way, but it's the easiest way. After a while, being familiar with it, you should be able to basically just cut it off in one go. I find that there is a minimum of edges left this way, and is pretty smooth.