while not a tool a possible solution would be to try this

The spawn inhibit range of fire/bench is about 30 meters. so put down 25 2x2 wood beams/floors t. Then open your map and zoom in as much as possible. Put map pins at both ends of what you have built. Thats your reference scale of 50 meters.

Then around and at each campfire and bench you have placed put a pin for it's location on your map.

Use ther reference scale to get an idea where the possible spawn points might sitll be.

if any fires or benches are more than 50 meters away from the other drop another campfire there.

Yes its a lot of work but you only need to do it once and it does the trick. I've got a large island in the prarie that I've completely secured using the method described above.

As to an actual tool I haven't found even a mod for it yet and I'm not well enough versed in programming for the Unity game engine to write one.

Hope this is useful to you. good luck