Real simple and fool proof!
Take a water jug like you buy water for a chiller. I used my dogs auto waterer jug as they've never used it. Then the rubber part from a turkey baster.(hey I cant help it I'm a redneck) You can make you're own container out of plexiglass if you like or a bucket etc.
Get a conatnainer that will fit and you can seal it up airtight. Put a bulkhead in the bottom of it with a barbed fitting and some poly tube from that. Place someting on top of you're sump or next to it, as long as it's above it. Run the tube down to where you desired water level is. When the water level drops to below the end of the poly tube, air will suck in and it will burp and then fill back up to the level of the poly tube. I think about 10-15 dollars tops will cover it.