ssc_build doesnt recognize .slx parts?

So I made a few basic shapes using solids in simscape, the files are saved as *.slx. I want to be able to use these shapes in future models, like building blocks, so I think a library is best, the only problem is I cant generate a library using ssc_build, as it only recognizes .m and .ssc files.

My current directory is in */Documents/MATLAB/testsat/ if that matters. I made a folder called +package in that directory, and put all my shapes/slx parts in there, when I run ssc_build.