Rule # One.
NEVER attach the gun to the chair.
The first thing an attacker will do is flip the bike over, and the gun goes with it.
A gun not attached to your body is way to likely to be lost or out of reach in an attack.
A strong-side holster is out.
Due to the sides of the chair interfering you can't get a grip on the gun without leaning WAY over or moving almost out of the seat to access the gun.
A ankle holster requires pulling the pants leg up to get to the gun.
Try doing this seated in a bike and you soon find out the bike interferes with pulling the pants up. Most have a leg strap to prevent the legs from sliding toward the rear and that blocks the pants leg.
Good carry methods:
The fanny pack.
It collects everything you need in one easy to put on and carry package.
You have the gun, spare ammo, wallet, keys, phone, etc all in one package.
In order NOT to take the gun with you requires the effort of removing it from the pack.
If you have to put on a holster, it's too easy to leave the gun home when "just running down to the local Stop 'N Rob.
Since many chair users wear fanny packs, and the black pack tends to blend in with all the aluminum and nylon of the bike, most people never even notice you're wearing one.
I recommend the Blackhawk Urban Carry 5-5-10.
This means you can draw the gun and fire five shots in five seconds, at 10 yards.
Of the many packs I've seen it's the fastest in action and the most comfortable due to the molded in padding on the back.
Next, a shoulder holster of the horizontal draw "Miami Vice" type.
These do require careful adjustment of the harness to properly position the gun (which few people ever bother to do) and they do take getting used to.
Once used to it, a shoulder holster is excellent.
Last, a cross draw, either a standard or a "Driving Holster" type.
Put on a cross draw positioned to the front and pull even a tee shirt over it.
Put your hands in your lap like most chair users do when at rest.
Notice WHERE your hands are........They're literally ON the gun.
All you have to do is lift the shirt with the off hand and draw the gun.
In use this is actually faster than all but a few standing shooters can do with any gun carry method because you hands are right THERE.
In most cases, people won't notice a cross draw carry even if they're looking.
Again, it all just sort of blends into the whole bike thing.