Does the shed have a wooden floor ?
What I did was, screw half a dozen long screws into the floor in a circular pattern leaving about three inches standing proud of the shed floor. Then got an old bucket and cut the bottom out so leaving just a plastic cylinder.
Bought big strong post-type ground anchor with a big eyelet on the top, put the bucket on the floor over the screws, positioned the anchor inside the bucket and filled the whole lot up with wet
concrete. Left the eyelet on top of the anchor standing proud obviously.
Then when dry you can remove the bucket leaving the concrete lump in place, I actually left the bucket in place though to protect the bikes a bit from the rough old concrete. Then big fat chain and lock through the ground anchor eye.
The advantage of this method is that the heavy weight, bucket of concrete in this case, is fixed to the floor of the shed and not ‘loose’. The scrotes have a habit of lifting the heavy weight plus everything else on to a small wheeled trolley and rolling the whole lot out of the shed/garage. At least this way that wouldn’t be quite so easy.
Not foolproof I know, but it makes me feel a bit more comfortable.