I ride a bagger Harley and when I first got it, I experimented with different ways to carry concealed and was concerned with A.) losing the gun, and B.) accessibility. With regards to (A.) I decided I needed a holster with a retention device. With regards to (B.) I needed to be able to get to my gun, if ever needed, with as much ease as possible.
After some trial and error methods, I was most comfortable with an Army Surplus Helicopter Pilot's Holster. I can carry a large frame semi-auto and the holster rides across my front lower left chest area. The gun (and holster) are secure, I don't have to worry about losing the gun because it vibrated out of position and a simple pull down of the jacket zipper allows easy access to the snap on the holster and quick pull-out of the firearm.
This type of holster can be purchased at most military surplus stores or CTD has them on-line.
May the Schwartz Be With You.