Not sure if you mean a 1/2" shank or 1/2" cutter? Either way, however, my guess is that it is a larger bit than is meant to be used in a trim router.
That is what I have done in the past also, works for me.JC - what will you be using the trim router for the most ?
as said above: a trim router should not be used when a full-sized router is appropriate.
if you can find some plexiglass, (or other plastic material) make your own base plates.
this is where "one size fits most" does not apply.
when I need a tight fitting opening, I drill the center hole slightly smaller than the bit diameter, lower the bit well below the base plate. and while the motor is running,
"gently" raise the bit up through it - turn the router off and unplug it. remove the base plate.
then with a round sanding device, enlarge the hole accordingly so the bit won't touch the base while in the normal operating position.
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