Looked at my HD2010 last night - you're right, but it's not a tight fit for a 16mm, and actually I didn't do a good job of centering the U3 1C I got in there now, so a shaved down copper 20mm may be better way to go.
Actually I got the HD2010 running at 4A, but my LightMalls C8 @3.8A w/U3 1C does much better in lumens measurements (995 vs 750), but the HD2010 throws better of course. This match's up well with rdrfronty's results here: https://budgetlightforum.com/t/-/11719. Seems like the HD2010 throws out a lot more light than what I'm measuring...