I recently replaced the elastomer on my 2005 Klein Reve V, which has same rear triangle suspension system as the OP, aka SPA. Used a 1.25" length of polyurethane rubber tubing that I got
here. I bought two 6" pieces about three years ago, one 40A and the other 60A. I didn't replace the original elastomer at that time because it wasn't in that bad of condition when I removed it back then. Cutting this stuff accurately is a real pain it the ass! Muffed up a few cuts using a miter box and saw before I figured it out: securely clamp the long section of the tube to the box, hold the other end steady without deforming it and patiently saw through it. After cutting, I sanded each cut end with emory cloth laying on a flat surface to get it reasonably flat and perpendicular. Anyways, below is what I ended with using the 60A tubing. Seems to work perfectly. BTW if you need this replacement elastomer part, you are probably far better off buying it
here than fabricating it as I have, especially if you don't have the tools to cut it.
Sorry I didn't post this earlier, but thought this info might be useful none the less. BTW, I love that bike, put a Shimano 105 ten speed grupo on it about 3 years ago.
Edit: I wouldn't trust using a piece of wood as a replacement for the elastomer as discussed above. There are HUGE shock impulses that go through that shock linkage if you are traveling fast when you hit even modest bumps in the road. Thus I imagine that over time the wood may fail and it could be rather ugly when it does.