I used to just use a pair of flatblade screwdrivers stuck thru a sponge and another sponge between them so they wouldn't flop around when I wasn't holding them both.
I later made a split-pipe fixture with a chamfer round-filed on the ID, similar to those used for a number of other bicycle-repair / installation tasks.
But after too many attempts to fix throttles being too-often spoiled by the magnets being loose and falling out, then having to try over and over again to put them in the right way (not marked, little cubes that *each* fit in six possible ways on some, little rectangles that *each* fit in four possible ways on others), with each wrong assembly requiring disassembly of the throttle *again*, and then putting them back in, reassembling, retesting, getting wrong voltage range on hall output, rinse and repeat...and fixing that only to then have something go wrong with the cable or the sensor itself from all the faffing around....
I gave up fixing them and looked for something more durable--now I use a really cheap metal ATV thumb throttle that pulls a cable-operated throttle body, which is actually waterproof-sealed (unlike the typical throttle grip or thumb). For those that prefer grip throttles there's plenty of motorcycle/atv/etc grip types that pull single-cables.
After several years the cable wore enough to begin sticking just a bit (even with lube) so ti doesn't always return fully to zero, so I need to replace that, but the thumb control and the throttle unit still work fine. (thankfully the Cycle Analyst v3 I use to convert and filter my sensors and inputs to make a throttle signal for my 2WD controllers has settings for all the throttle voltages, to compensate for whatever a throttle actually puts out and transform that into what a controller needs...so I have not had to get around to replacing the cable yet).
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