I don't know if is possible to "crop" just the output crawl.
You need to add the overlay items.
Using the mask you will mask/hide all other itemes under mask's Z level.
By default the crawl will be on Z-Pos = 0.
The background can be on Z-Pos = -5
Mask can be on Z-Pos = 1
Put all overlay items on Z-Pos = 5
#XPression