Shims won't work; or if they do, not well or for long. The essence of the fit of a taper is that that there is a large surface area. If you get the small end of a tapered axle to fit - with shim - into the un-tapered crank, the whole of the fit is a line of contact at the end of the shim. The only way you get any area is by distorting that end into a wedge, but you still have only a small fraction of the area of a matching-taper fit. The crank will squirm; if you only have one flat shimmed it will also wobble; and that end of the shim will quickly be permanently deformed and the fit is then loose.