I’d like to use my retired Ridley Supercross more, but it has no bottle bosses.
If it were aluminium I wouldn’t hesitate in fitting some Rivinuts, but it’s scandium and seems paper thin.

Scandium is actually just shorthand for an Aluminium alloy which has Scandium in the mix, typically a minute amount (Scandium acts a grain size reducer with Aluminium, creating an alloy with a higher strength to weight ratio than normal Aluminium alloys, thus allowing the tubing to be thinner). So in standard circumstances the tubing can be dealt with like standard Aluminium alloy, in that it can be welded and drilled etc. As you say though you’ll just have to rather careful due to the thinness of the tubing.

Edit: Just a thought, another slightly bodge like way of doing thing would be to drill the two holes in the frame for a cage, then use a cable tie or two to go through said holes and cage eyes to nail them together. They’ll still be a little flex but it should hold fast.

Bottle cages were fitted to scandium road frames using riv-nuts. Just make sure you use a sharp drill and deburr the edges

Zefal ‘Gizmo’ zip on bottle bosses…

Ziptie/Velcro solution would be the easiest fix, something similar to the above. You could always put some clear invisiframe type sticker underneath worried about marking the paint.

https://www.alpkit.com/products/love-mud-bheesty any good yo you?

Pretty recently I got some bottle cages from TK Maxx which came with a split clamp which might be useful, Same as the “Crazy Cage and Clamp”on this page http://nordicgroup.us/cageboss/ the page also shows a gazillion other options.

Has anybody ever tried just putting a small hole at top of downtube, bunging up the holes at the bottom and then filling/drinking water directly out of the tube using a long drinking tube?