Vixen dovetail and an ST80

Hi all,

I've recently bought a Vixen Portamount for my ST80. Lovely mount, I
really like it (took it on holiday recently).

The problem I've got though is connecting the ST80 to it. I've got a
Vixen dovetail plate which I've managed to screw on to the OTA by
drilling two new holes in the dovetail but it's not very satisfactory -
the dovetail is flat on the 'OTA side' and so doesn't mate with the
tube properly. I've fudged it with some rubber spacers at the moment.

Is there a proper way to do this? Can you buy dovetail that mate with a
round tube? I've hunted around a bit but I can't seem to find anything.

Thanks for any hints.....

J.