One could always make the vertical tubes with very thin tubing. Perhaps get 1mm ID tubing and use that for the vertical parts, then use wider tubing for the coil at the bottom. That'd get down to the point where the difference in output due to depth was negligible.
There's not much bother in topping it off once and a while. Once a month sure beats once a day!
Also by topping it off, you won't shock the tank with a change in your pmdd recipe.
Otherwise you could always do this:
It's another container, but DIY plant geeks already have a PHD in glueing tubes to the caps of bottles.
I should point out that the bottle needs to be able to withstand negative pressure, thick plastic or glass - preferably clear or translucent.