It's likely bad practice but I do all of my (personal) external files as .h (with decl. and defs. inside), and header guard them.
If I understand it correctly, it's essentially cut/paste into the one file and should only come back to bite me if I want to use these functions from inside a different translation unit?
Maybe it's simply just safer to do the .h .cpp anyway though, so noted and thanks! 
Appreciate the help with this @gfvalvo @J-M-L so if you were approaching this issue you'd not change/add anything to this?