I believe the best approach, especially if the tank is not set up, and you know the glass isn't tempered (easy test for this), is to drill the tank and put in an internal overflow. I started with an external HOB overflow, with safe-guards, and after two floods, I was sold on a drilled tank.
You have a number of after market choices with overflows, all smaller and less obtrusive than the typical corner overflows found in reef-ready tanks. I'd highly recommend installing one of these overflows.
Now, if you still wish to pursue the external, and have DIY skills,
HERE is an example of a float switch (with backup, which I'd highly recommend) that would control the return pump. This potentially could be installed in the sump and detect too low a water level and shut the pump down. Most (if not all) reed valve float switches can be switched for normally open to normally closed by flipping the little float around.
If you don't wish to make your own, I've included a link in the referenced link above to a kit that will do the same thing (AquaHub Premium kit).