How to force a solid layer

Hi!

What I'm trying to do is quite simple, a sign with letters on top of it.

Even though the letters are separate objects in the assembly and use a different color, Bambu Studio tries to combine the shells, which means the topmost layer of the sign plate isn't completely solid, but has "holes" with sparse infill where the letters will be. This makes that layer look worse than it needs to be, it would be cleaner if the printer just completely printed it as a solid top layer without interruption and place the letters on top of it.

Is there some way to force the slicer to do that and prevent it from "combining" the sign plate and the letters?

Thanks a lot in advance!