I've been waiting and hoping for MakerBot to fix the Thingiverse Customizer before this contest ends, but I am going ahead and publishing what I've done to date.  The OpenSCAD file works fine IN OpenSCAD, but as of today (16 Feb 2018) I still cannot test it within their Customizer UI.

The .scad file is attached to this project.  I hope someone else out there finds it useful.

I'll continue to check in with Thingiverse and hope to see a fix for their system soon.  Once it is fixed, I'll go back to making this part Customizer-friendly.