We have just fitted the second tank and I wanted to share my acquired knowledge. So, for all who are looking for a way to fit the big tank on the SE (model SP46) I have posted parts list, instructions and pictures with measurements here:
http://beddha.free.fr/DR650SE/TankAdapter/
Sorry about the lack of quality of the pictures: the parts were scanned, not photographed. That is cause Canon can't be bothered to repair our camera under warranty. It's been almost 4 months...
Feedback is welcome, in case of errors, improvements...
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Salut from Southern France, the bikers' paradise,
Peter.