At the rewinding place I used work ,two part slow curing epoxy was the go to for jobs like this . It remains a pourable liquid for hours after its mixed .
Using a little less hardner will leave the cured epoxy with a little more elasticity which might stress the windings less due to thermal expansion/contraction . Mostly you see toroids mounted with a large washer top and bottom , insulated with neoprene sponge to prevent crushing the windings together . Of course it goes without saying making sure the mounting bolt cant make contact with the top of the chassis , and completing a one turn shorted loop is a good thing , sparks and molten metal flying around inside the chassis most likely wouldnt end well .
You could embed a nut and some plastic tubing like Shaba suggests , but leaving enough space so the mounting bolt ,nut and washer are recessed in the middle of the doughnut works fine also .
Mostly what Ive seen on toroids with epoxy infill is first the centre is filled , allowed to cure ,then the bolt mounting hole is drilled afterwards . With liquid epoxy and the nut embedded theres always a chance the threads could end up clogged , you would almost certainly destroy the transformer if you ended up having to hack out the solidified epoxy.
Like in the first pic below , the top of the bolt ,nut and washer are reccessed and cant come into contact with chassis under any circumstances , in the second example , if you dont have much clearance between the top panel and contact is made you create a spot welding machine .
