Yeah, I've done it many, many moons ago. I cut up an old Coke can and used trial-and-error to get the fit. It worked just fine. Not the most asthetically pleasing solution, but it will work.

I thought I was off by .2mm, but it turns out I was .4mm off. Based on my experience, the issues I see are: (1) the shim I ended up with was pretty narrow, so if you are only off by .2mm your shim will be very narrow; and (2) if the top of your seat lug has a shaped profile (as opposed to a square-cut profile), you will have an appearance issue when you bend the shim to hang it on the seat lug so it doesn't fall into the bootom bracket.