Star fangled nuts do not offer any ‘steerer tube support’, they would not have stopped this happening, SFN are designed to deform, that’s how they gets squeezed into the steerer, they are not structural.
The steerer is hard steel, the stem is soft alloy, it’s easy to guess which one has deformed and is now scrap. If a stem has stretched and deformed like this then using coke can to shim it out is a recipe for disaster. All that will happen is it will deform again. Just buy a new stem. Coke can shims are fine for seatposts if you’re a cheapskate bodger where the only thing at risk is your arse but not for stems where loosing control is a sure fire face plant.
For the sake of ten pounds and your teeth buy a new stem