I've repaired two airboxes, one on my Spider and one on a mates 156, so I'm a little hazey to what I did with which.
But I think it went like this.
The 156 I removed the lid and carefully drilled a hole through each hole in the lower part of the airbox, the drill bit end will come out inside the airbox, I sent longer bolts upwards from inside the box, through the holes, lid back on and span the nuts on top of the lid. I drilled the holes out carefully, so not to be to large, so they'd grip the new bolts, otherwise they'd fall in the box before you got the lid on, as you can't get hold of them inside the box.
With my Spider, I removed the lid and drilled out the bolt holes in the lower part, I drilled all the way through as before, but unlike the 156 the drill bit end doesn't end up inside the airbox, but outside under the lip, then you can put the lid back on, run longer bolts down through and reach under the lip on the outside and spin wing nuts on.
Before you put it all back together, vacuum out the swarf from the airbox.