I found that WHERE the spring bears makes all the difference in the world. If the tip of the spring bears as close to the pivot screw as possible, it almost doesn't matter how stiff the spring is, the sear has so much leverage that it depresses very easily.
Shortening the spring a bit so it bears farther from the pivot will increase the pressure required to depress the sear tremendously.
Not that you are talking about shortening the spring; just be really careful not to take any metal off the tip while you merrily remove blade thickness.
By the way, these springs are really easy to make. Just in case you flub it up. Been there.
Tom