You could try to read my guide that is also in my "signature":
https://github.com/onekk/freecad-doc
With scripting, you could design almost all with parametric things, and also define maybe some joints, to join pieces to make them longer than the "3d printer bed".
Once accustomed there is the appropriate section (Python Scripting and Macro) where to ask for advice, but please post some code so it will more easy to help you if there are problems.
Hope it will help.
Regards
Carlo D.