For 68 spokes, you would need either 4 or 17 evenly spaced bolts to go thru the spokes to make any kind of adapter work. You could do this by getting an undrilled sprocket and taking it to a machine shop where they can drill the holes evenly spaced, then make up an inner ring, drilled, then cut into 3 segments like the typical rag joint by using 2 rubber rings that are drilled the same way.
If you can mark and drill the holes yourself, you could save some money, but if you're trying to drill 17 evenly spaced holes thru a steel sprocket, it's going to be time consuming to say the least, you could probably drill 4 evenly spaced holes, but a rag joint wouldn't be very stable with that few holes. Otherwise, you would have to have a sprocket adapter custom made similar to the manic type but that would be quite expensive at the machine shop.
The cheapest and easiest solution would be to get a 36 spoke rear wheel, but it can be done if you got the resources (or money) and want it bad enough.