"How did you finally resolve the problem? "
With the eraser on the back string, as described. The added mass of the eraser drops the pitch of the back string. You start with it in the middle of the after length, which gives the lowest possible pitch for the amount of weight on that string. If it's too low you move it toward one end or the other, until you get the right pitch. Moving it up toward the bridge makes the given mass more effective at controlling the wolf, but may also alter the tone of the fiddle more. Moving it back toward the tail piece has the opposite effect. Using a piece of rubber, such as an eraser, adds some damping which helps it to work and spreads out the workable frequency band, so the adjustment is not so critical. 'Cello wolf note supressors need more weight, so they use a metal tube that is slit down one side, with a slotted rubber plug in it, and usually have a set screw to fix them in place once you've found the right spot. You need to reinstall it every time you change the strings, of course.
There are patented 'wolf eliminators' that can be glued up inside the top near the bass foot of the bridge. Since most 'cellos have a wolf at more or less the same pitch they can be pretty standardized, with, maybe, a small tuning range. A acquaintance of mine who works in a violin shop once got a call from the Customs Service about them. They had ordered a box from Germany, and when they arrived at the airport the customs folks noticed the label. What did she intend to do with these devices? Was she aware, they asked, that wolves are an endangered species? It took a while for her to satisfy them....