Reply by Loren

Making unusual mortise and tenon joints is
not usually a good way to reduce costs.

You can do it like you’ve drawn (I’m confused
by what you’ve got in mind with the 1×3s
but that’s a different issue), but you’ll need
a way to pull and fasten the joint snug… a
peg, wedge or perhaps a metal doohicky.

Excavating that mortise into end grain is
going to be a pain.