There is an ASCII text format of STL, and you can write an STL by hand in notepad.

I learned how to write STL file format here.

I think this is the simplest possible STL.

It's a tetrahedron.

It is written like this:

solid m
facet normal 0 0 0
outer loop
vertex 0 0 0
vertex 1 0 0
vertex 1 1 0
endloop
endfacet
facet normal 0 0 0
outer loop
vertex 0 0 0
vertex 1 0 0
vertex 1 1 1
endloop
endfacet
facet normal 0 0 0
outer loop
vertex 0 0 0
vertex 1 1 0
vertex 1 1 1
endloop
endfacet
facet normal 0 0 0
outer loop
vertex 1 0 0
vertex 1 1 0
vertex 1 1 1
endloop
endfacet
endsolid m
 

The normals are wrong (they are supposed to be vectors perpendicular to the defined facet), but if I wrote the correct normals that might make it larger, and you don't need the normals to be able to print the STL.

Print in place without supports, scale up for best results.

I look forward to seeing your makes and please comment if there is a simpler STL.