Depending upon the filter size of your lens, why not try the Nikon 5T/6T (used, on eBay), a 62mm 2-element close-up diopter which doesn't degrade the image too badly in practice or the Canon 500D, if the filter size is larger?
I use the 5T/6T on my 70-300 and the 500D on the 70-200 regularly. Though I have "real" macro lenses, up to 200mm, these are much more convenient and do a swell job as far as IQ.
Either of these will be less expensive than even the most beater 50-60mm macro lens, allow a greater working distance, and, on a zoom, allow easier framing than a fixed-focal length macro lens.
As always YRMV and perhaps, it won't be sharp enough for your application.