I am not sure about the exact answer, I suppose I would go for installing an anchor point concreted under the floor. One thing to check if this applies is whether your house / bike insurance excludes bikes kept in sheds (some do regardless of security measures).
If you have decent bikes and are spotted riding around, you're at greater risk of burglary.
Pick the site of your shed. Get some steel cable and attach it to a big steel plate. Dig a foot-deep hole and put the steel plate at the bottom of it. Fill the hole with rubble and concrete. Cut a hole in the floor of the shed and put it on top of the cable. Lock the bikes to the cable with the biggest, heaviest lock you can find.
Or just put a basket on the front of your bike - it'll never get nicked.