How to carry a concealed firearm on a bike?
A thread on this subject on Bikeforums ran to pages and pages...   Some expressing utter horror that a bicyclist would feel the need to be armed, others ridiculing the whole idea, others with decent suggestions.
I'm a cop, I know that numbers of cyclists ride in conditions that are less than optimal.
We have had armed robberies and muggings of cyclists here in St. Louis, usually the bad guy is after the high-end bike.
As well, mountain bikers have been attacked by cougars in Western states.  Bicycling magazine detailed several such incidents while I subscribed.   Rare, but possible.

First up.  Having the weapon concealed on the bike is rather a bad idea if the intent of the bad guy is to steal the bike...  Your first indication of anything wrong might simply be a pedestrian knocking you over at an intersection.  Now, he's got your pistol too...

Unfortunately, cycling gear isn't much designed for concealed-carry.  I have a little Taurus 2" 38 special as an "off-duty" weapon.  Drops right into my jersey pocket.
Sure, it "prints" there.  But then, I'm a cop...I don't care.
A reasonable idea is the "fanny pack" (I've just been informed that Aussies call them "bum bags"...).
As far as I know, you can still buy a fanny pack that is designed specifically to carry a firearm....