Oh, I get it... sorry, I had not considered that. What you need is not a longer cotter, but one with a different taper. I don't know how you're going to find that, though. Therefore:
I would cut a shim the size of the taper on the cotter. Steel from a coffee can might be thick enough; or you might need two or three layers, I don't know. Make sure it's smooth and flat and fits perfectly in the flat spot between the cotter and the spindle., then proceed as normal.
Yes, this is a cheesy workaround, but as long as the cotter is inserted with enough pressure, it should be okay. Especially if you don't expect to ride the bike a great deal. I'd carry the cotter tool with me when going on long rides.
No worries, I have not taken a cotter crank apart since I was about 16 so I am not sure of anything. I do see the taper you mentioned and will compare that to a search vs size. I may also measure and email some of the ebay sellers and ask for sizes. If not I shall employ the suggested jury rig.
