Sorry Andrew. It's a smaaall source of my income. Only moded 8 Telrads since I designed the circuit in 1999, & I don't have a Telrad.
A couple of our W.A.A.C.ers use a 2 litre fruit juice bottle or a plastic sheet, folded over the top. Seems to work for a while, but not well when there's really heavy dew. (cheapskates)!
L.