BTT SFS Back Cover

This is a plain back for the BigTreeTech SmartFilamentSensor. I know that a stl file is available directly from bigtreetech on github. This is made available with the scad file at https://github.com/mr-ice/3dp_btt_sfs/blob/main/sfs_back.scad (uses libraries also available at my github).

This is a jumping off point for making mods that will incorporate my SFS.