Congrats on stopping your leaks!
How much water will be going over the spillway? Is it just an earthen emergency spillway?
You can always just put up some chain link fence and make it "progressive". By that I mean put 8" of fine mesh fencing at the bottom of the chain link so you don't lose any valuable fingerlings. If the spillway flow rise higher than 8", then the chain link will hopefully retain your larger fish. As the depth of the water flow increases, it will start exerting a LOT of force on your fence posts. They need to braced t-posts, or actual driven/cemented fence posts to survive.
The pressure on the fencing will be even worse if floating sticks and leaves can accumulate on the fencing.
If you are usually going to receive smaller flows over the spillway, you could still easily install a siphon system at your pond. (Especially if you have a low area behind your dam.)
You can put a screened inlet in the middle of the water column and set up an automatic siphon system to start moving water over the dam BEFORE it reaches the spillway elevation. That will save all of your fish, except for the heaviest rain events if sized properly for your watershed.
(PVC gets much more expensive and much more difficult to seal perfectly as you move to larger diameters. You cannot have any pinhole air leaks in a siphon system.)
Hope that gives you a few ideas, since you are the one with actual eyeballs on the project.
Now that you have reduced your worries about your pond running dry, lets not add the worries about losing your fish!
