Here is an older thread talking about siphon breaks. There is also a drawing included so you can see what we are talking about.
Do a search on "siphon break" as well on this site. There is a ton more information about them.
You can also buy a check valve. Surprises me that I never hear of anyone using them. I guess drilling a hole is cheaper. MarineDepot.com sells them.
Yeah, you could do this I guess. I run 1/2" PVC
as my return. MarineDepot sells a 1/2" check valve for $16.99. That is without shipping.
Your right, must cheaper to drill a hole.