Printing molds isn't tough, though you do have to dial in your printer settings to get parts that are up for the job. The most important thing to remember is that flaws in your printed parts will show up as flaws in your silicone castings.
The parts pictured above are the ones you'll need to print to replicate the locking plate. These are "LOCKING-PLATE_pattern_bathtub_mold_02.STL" and "LOCKING-PLATE_sprue_bathtub_mold_02.STL" in the "Locking Plate for Flat Pack Camera Arm" Thingiverse project.
The things to look out for are stringing across the print, cracks along the layer lines, and underfilling. Your printed part needs to hold silicone in without leaking for a few hours until it cures, which means the printed layers need to be watertight. I find that taking it slow and printing with at least a 1.2mm wall thickness gets me dependable parts on my Ultimaker 2+. You'll also want to set things up to prevent warping, whether that's using a raft to keep your part's corners from lifting or enclosing your printer to avoid drafts.
Once your print is complete, clean up any support material or stray wisps of filament. A light blast with a heat gun can melt and clean up fine filament hairs.