Is that the one with the hexagonal inner shaft that has the head pinned on? I've broken one of those myself.

We broke a "Post Moderne" seatpost two weeks ago. A field repair got us home. The metallurgy or the design seemed to be lacking in strength.

So.....looking around the house, with ultra-strong clamp and shaft as requirements, I found two that seemed to have come from the same manufacturer, though they have different decals on them. I got one from Nashbar, and perhaps you should try that, in combination with a shim from Precision Tandems.

This seatpost has the typical cheap Asian head, with the mating serrations on matching curved surfaces, held together by the relatively massive 6mm bolt(!). Dissassembly on one occasion found a strong spring and an elastomer, all for a low, low, Nashbar price.....it seemed to work well with very low stiction and a tolerable preload, though I have considered pulling the elastomer out and replacing it with an appropriately sized piece of tubing and some washers, to get the preload way up to provide a stiffer pedaling platform. Tell us what works for you.....