This might be the solution if I can't find a 5-10mm conical spacer:
A Veloflyte Headset Dust Cover + one of my carbon spacers (1-5mm or 2-5mm) on top of it.
I was concerned about the dust cover being a weak point, but it comes with metal shims so support it underneath.Veloflyte Headset Dust Cover Bontrager Standard Carbon Spacers
Top cap replaced (previously, alloy OEM cap) with no-name carbon (9g) and that has been replaced with the Bontrager carbon top cap that came with the Bontrager XXX Race Lite OS Stem. I've also switched the bolts (3g OEM for 2g bolt) and starnuts will be exchanged tomorrow "if" the one I have now is actually lighter than OEM).
With original 2.5mm spacer and top cap (I don't know if you can see it in the pictures, but in person, the more tapered Bonty cap and 1mm spacer make a HUGE difference, not to mention that the carbon looks a lot nice than the matte black Al cap):
With 1mm spacer & Bonty top cap. I'm going to file down the hard edge a bit to soften it up in order to taper it to meet the cap: