A few weeks ago I bought a Pletscher rack for Hedwig. Unfortunately I discovered that I was going to have a hard time getting it attached to the frame, as you can see here:
When the rack is sitting level, the holes sit right over the tubing. Hedwig's aluminum tubing is just a little bit bigger than the steel tubing on other bikes (like my Fuji, which the rack fits fine on). So, since I was trying to mount the rack about half an hour before I needed to leave for work, I just mounted it as low as it would go, which just barely let the screws clear. It put the rack at an angle, which at first I was fine with.
I just added my bag, which wanted to slide under the seat some, and off I went. It's been that way for a couple of weeks now, bothering me a little more everyday and yesterday I finally had it. The problem was that because the bag wanted to slide under the seat, it was also being semi divided in the middle by the narrower bracket end of the rack. The bag wanted to hang on either side of it, making it difficult to get things in and out of it. So I brought it into my husband's shop and we brainstormed.
We tried several things, some involving wooden blocks, and drilling out new holes for the original hardware. None of those ideas would work. Then we noticed the little tapered points up near the seat and decided to try just zip tying there.
And it works! So long as I don't try to haul enormous loads on this bike, I think that this is going to be just fine. We may switch out the plastic zip ties for metal, or some other metal wire intended to support heavy loads (and not stretch). Now, since the rack is level, I don't have room to put the bag on the back of the seat and still have room for my rain coat on the rack, so I hung my bag from the side of the rack. Which I really like, and am formulating a little scheme to make some custom panniers.
With zip ties. :)