If anyone is still interested in diopters, I have a couple of spare +0.5 diopters with 72mm thread.
These are no achromatic doublets; they are single element, but are excellent quality and personally I use them anytime on my Iscorama (with results comparable to the all time classic Tokina +0.4 achromat).
On an Iscorama +0.5 will enable you to get closer than the 2m minimum focus distance of the Isco, in the 1m to 2m range. A second +0.5 stacked will enable you to get a nice close up in the 0.5m-1m range. Usually I have all range covered with only two of those.
As a bonus, these are thinner than the Tokina single element ones, so you don't get vignetting even if you stack 2 of them.
The 72mm thread is nice, since they directly fit on the Iscorama, or on the redstan front filter clamps. I have also used them to get a close up of an eye, with a Canon EF 135mm f2.0 L lens.