Have you thought about just using a canister filter with the overflow?
That's not what they're asking, they basically want to make their overflow a kind of filter.Yes, you can plumb a canister through the drilled holes.
Make sure you can turn off the flow for servicing the filter. Ball valves in the tubing, if the filter does not already have that covered.
I would run it a bit to be sure there is always good flow before placing the heater in there.
Put all that media in a coarse mesh bag (around here onions and avocados come in those bags, and they will hold up in an aquarium) so you can remove it for cleaning. Be ready to vacuum the bottom of that chamber, it will get gunked up.
I estimate my overflow will allow for 18 cubic feet of evap before it runs dry
that would be like 3' x 3' x 2' or 4' x 2' x 2.25'?