I have a pair of factory Recaro seat bases that allow easy installation of aftermarket Recaro's into the C5 platform. These came originally on the European C5 A6 and same era VW Passat. They have mechanical sliders, and are designed for non-power seats.

It would take some fabrication to deal with the factory electrics, but generally these bases allow you to bolt-in Recaro Sportster-style seats with OEM parts.

These parts are really hard to find in the US, and it took me a while to track down a set and have them shipped from Europe. One of the bases is missing the upper seat mounting brackets (see pictures). The brackets are pretty simple, a piece of flat bar bent on the ends with holes drilled to adjust the seat height. It would be pretty easy to fabricate something similar.

The Recaro part numbers are 86.23.17 and 86.23.27.

Located near Portland, Oregon

$600 for the pair, including shipping to the lower 48.