I just used 2 bulkheads thru the side fof the tank. I'm mot entirely sure how you would neatly and cleanly "screw in a fitting" into a bilkhead, hard pipe it to where you need it, and in the rare event you needed to remove it, it would need to be cut, allowing that $@#$ into you tank/overflow to circulate...
Like I started to type, I placed my bulkheads up high enough that I also used a siphon break, and then there is the check valve... you could run a double check valve, if it really worried you, but I can show you that when you stop this weekend. The cheapest place to find a valve is at the hardware store by the sump pumps... but you want the ones with flapper, not the spring loaded ones, these slow the water down to much.
I've seen the ALL GLASS style returns, the ones that the returns reach out of the overflow box like a octopus... those are nice, but the return box needs to be large enough to sevice them inside the box...