Nothing is completely foolproof but you can make it hard to steal.
This is what I did for my Honda 3000 but it is on a trailer.
Welded a frame together and bolted it to the trailer deck.
Used two different types of screws.
Also used 1/2" carriage bolts with two different size nuts. This pic is the underside of the trailer. Note the little pieces of pipe. I someone would need two different size deep well sockets to get to the nuts.
Lastly, a cover goes over it and bolts to the iron frame.
Not impossible to steal but you better bring a tool box.
Zulu