Best way to split up an STL?

I've got a MP Mini Select v2, so not the largest print bed ever, and I'm trying to build an enclosure for it using the parts in this tutorial: https://www.thingiverse.com/thing:2864118

I hit a snag because some of the STLs have 5-6 parts to print in a single STL and they won't fit the bed. Specifically the Top and Bottom Enclosure STL files are giving me grief.

I've tried using both freecad and Blender to figure out a way to select enough parts to remove that they'll fit my bed and I can get printing, but being a complete novice at 3d modeling and those apps, I've had 0 success.

Any suggestions/advice/help as to how I can just delete certain parts of the print so I can do each part in 2-3 prints instead of one large one?