I would think that could be tough to do safely and acurately on a table saw. Maybe lower the blade all the way, position and secure the 2x4, then raise the blade to cut from below? I think a miter saw would be a bit more ideal if you can get a hold of one.
/is not a professional woodworker for very good reason
I'll be building one myself in the next couple days. I've been trying to figure out the design in my head today.