you're on the right track!
you don't need the extra chamber on your inside "box"
and the chamber "baffle" or weir actually on the outside box needs to be "taller" - this is what sets your tank's water level.
it also prevents the siphon from breaking during water changes and the like
depending on how many GPH
you want to flow, you may need 2 or 3 siphon tubes and drains ... I had a calculator for this at one time- i'll try to dig it up for you
going great - keep it up!