I got myself three of the "Universal Plate L" from pololu (or rather, as it turned out, from RelChron).
Though these plates cost me an arm and a leg (one plate goes for 9.93, plus shipping of 12.50 Pound to Germany!), they are exactly what matches to the Tinkerforge mounting holes.
Durable plastic, 3 mm thick. No hampering, all my bricks and bricklets settled nicely. Except, of course, for the LCD 20x4 bricklet. "Non standard" hole distance!
If there was a way to get them a little cheaper, I would rate these plates ******* (7 stars).