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You would have to change the value in the source code, as they do exist but they do not show up in the customizer. Openscad is not that hard to use, I did a very vey beginner's guide to it in case you do not know how to program: http://www.tridimake.com/2014/09/how-to-use-openscad-tricks-and-tips-to.html

In this case you would have to tune the proper variables near the start of the code, press F6 to recompile the object and save as STL. Above are the default values that may be of interest to you:

// Thickness of the internal arm in the 3-arm mount part
gopro_connector_th3_middle= 3.1;
// Thickness of the side arms in the 3-arm mount part
gopro_connector_th3_side= 2.7;
// Thickness of the arms in the 2-arm mount part
gopro_connector_th2= 3.04;
// The gap in the 3-arm mount part for the two-arm
gopro_connector_gap= 3.1;