The cheap way out is to use a lever to amplify the sensitivity of your present scales - tape an eye to one end of your landing net handle or similar,and carry a matching hook to attach to a handy tree/fence/bankstick; rig up a hook for the other end, to take the weighing bag/net, and attach a loop for the balance hook betwen the two. If the balance hook is half-way between the hooks, you'll double the sensitivity of the scales; if its one quarter of the way from the fulcrum (fixed end - tree etc) to the weigh-bag youll covert querter-pounds into ounces;if your maths are up to it you could even covert imperial to metric.Calibrate the sytem with kitchen weights to convince yourself and your buddies!
Conversely, if you have a thirty-pound pike and twenty pound scales, you can double their range by putting the balance at the end of the lever and the fish in the middle.
The BRFC might not accept it, but if you think you've caught a record, phone for a reporter with the right scales!