Symbols (at least raster symbols) cannot be trimmed. What is normally done in CC3+ is to just hide what's outside the map border by placing a white polygon on top of it. This is usually done in most templates by placing a polygon on the SCREEN sheet.

If there is a polygon there already, just extend it to cover up everything.

Also, when exporting, you really should have the restrict to map border and crop to aspect ratio on, so you don't get things outside the map in your export.

I've written a blog post with explains the RGB Matrix effect in more detail.

Did you remember to run DELAYDRAWSYM and set it to 0?

There you go.

I am doing the livestream on thursday, and thought I would talk a little about ice caves. Stream will commence at 1900 hours alpha time. I am sure you can convert it to your own timezone somehow, or just change all your clocks to be in my timezone. Or for the easy option, just go to the video below and see what YouTube tells you. There is even a reminder button.