Have you considered getting one custom made to fit in place of the rearsight? I'm not familiar with the Tau but I would have thought the rearsight assembly will mount on with one or two screws.
I had a weaver rail custom made for my LP10, see here:
http://www.airgunbbs.com/forums/showthread.php?t=390852
Since getting this I've decided I'm not keen on over-barrel mounting of optical sights on airpistols, and much prefer them further back like this!
HTH
Adam
“We are too much accustomed to attribute to a single cause that which is the product of several, and the majority of our controversies come from that.” - Marcus Aurelius
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