By not bothering with it ... instead:

1/Cut two strips of 3mm thick flexible plastic.
2/Drill holes in both that match the bolt pattern on the fork legs ... countersink one set of holes.
3/Bond the strip that isn't countersunk to the outside of your dry bag.
4/Once dry, drill through the holes and dry bag.
5/ Using countersunk M5 capheads, fasten the drybag to the fork leg from the inside. Use the other plastic strip on the inside of the drybag, so the drybag is sandwiched between both strips.

Hopefully it'll work. There is the issue of putting holes in your drybag but I really don't think it'll be an issue with regard to water ingress but we'll see.

What I never understood about anything goes cages is why are they mounted to the back of the fork leg. If they somehow got pushed into the wheel then the wheel rotation will pull them further into the wheel. If they where mounted towards the front and got pushed in, then wheel rotation would push them out from the wheel. Or is this not a problem and having weight centralized more important.
Good luck with the fix Stu, let us know how it works out.

good point Ray!

Stu, surely the bag will flop about badly? Still interested in seeing how works out though.

You can continue to follow Salsa's lead and put some toe straps/bungies around the fork/bag to hold it tight.

Any verdict on your fix then Stu?