The Dr Foster & Smith catalog has a flow meter that would work perfectly in your situation.
Just found it: page 18, F&S Aquarium Supplies 25 years... at the bottom,
part #AGN-25783 $ 7.99 for up to 250 gph
part #AGN-33040 $ 21.99 for 200 to 500 gph
Attaches inline to your output tube.
Hope this helps...