Projecting Egde of STL Part onto a Plane

[ClimberKyle]

Hi all, I used photogrammetry with the Polycam app to 3D scan a climbing wall and now I am trying to cut out the holds of the resulting STL file and make indoor climbing holds out of them on my 3D printer. My big hangup is trying to make the edges of the hold in the STL file project nicely onto a plane behind it to create a nice backing to rest on the climbing wall once printed. Any ideas on a good way to do this? I have experience with Blender, Solidworks, and Meshmixer. Thanks!