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I'm gonna go with the eternally helpful 'it depends'. Carbon steerer? Number of spacers? Intended riding?
In general, I wouldn't be concerned about replacing a spacer with something that is just going to keep a bag spaced off a headtube. I have a colleague who machined a pair of spacers, one to sit above the headset, and one which sat under the crown race to create mounting points for a front rack. This was on a light weight gravel bike which he then thrashed around Georgia for a few weeks. He didn't have any issues.
On top of that, people have been using spacers for canti brake stops and they haven't been killing forks as far as I'm aware.
The trouble with your design will be having a stiff enough structure to bolt on to your clamp. I presume you will need something which hangs down, you could maybe brace it back to the bar to create a truss?
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chiroshi
I've been thinking about DIYing a decaleur to prevent my handlebar bag resting on my cables as they loop from underneath my bar tape down to the frame. This won't be supporting the bag as such, just acting as a standoff. There are a few products that already do this, including the Restrap Bumper Bar and this from Tim Tas Rek, which actually is a rack/bag support.
I've been wondering about using a seat clamp with rack eyelets with the lip ground off as a mounting point. Specifically this model, which is flat. The 28.6mm version could replace a headset spacer or the 34.9mm version could be fixed around existing spacers. Are there any potential issues with this?
From a quick trial with a 34.9mm plastic light bracket clamped around a spacer it looks like the spacer can be turned quite easily (not surprising since the stem doesn't apply much force in pushing it down onto the headset), which feels a bit disconcerting. Also I worry that there might be a slight fretting action as a bag bumps against a standoff attached to it.
Alternatively, a 28.6mm seatclamp wouldn't be able to turn as it would be tightened onto the steerer tube (after the headset preload has been set and the stem bolts tightened) but, again, I worry about a fretting action of it being bumped by a bag. Is this something to worry about, or are the loads negligible compared to the loads impose by normal riding?