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I reckon I could sort something using a System EX quill adaptor and an L bracket, as long as the interior diameter of my steerer is standard for a 1 1/8" quill (it's threadless)
TvH-
as long as the interior diameter of my steerer is standard for a 1 1/8" quill
Unlikely, as steerers made for quills are usually butted, the wall thickness needed to fit a quill stem is a bit on the thin side to be used at the crown.
A quill for a 1⅛" steerer (a rare beast, but they did exist) is 1", the quill for a 1" steerer is ⅞" (22.2mm) which is an ID you might actually find at the bottom of a fairly chunky steel 1⅛" steerer, but you could shim it if the ID is a more typical 24mm. The SystemEX thing will be ⅞" with a shim to bring it up to 1", so you'd be looking for a thinner shim. Seems like a lot of hard work for a worse outcome compared with drilling the crown.
gbj_tester
Prime candidate for DIY.