I had a welding shop for many years. (I still build lots of things for myself, friends and family) I have built all kinds of things to prevent theft. You need some type of cage that sits down on top of the generator and locks. The lock has to be inside some type of metal tubing. This renders the saws and bolts cutters useless as they will not go inside the tubing. It can still be cut with a saw but the thief will have to make multiple cuts and will take a lot longer and if he only has one battery it will run down before he get it loose. Now if a thief come along with a cutting torch he is going to get it.
Go to a store that sells contractor job boxes, take a look at how they lock and you will understand the concept. Here is a link to one. It could not be solid if the generator ran inside. If you look at the front, the locks go up inside the holes on the front. Saws and bolt cutters will not get to them. You could always use a box like this and cut some holes in it. This one is 48 x 24 x 27.5
Job Box