I gave up on the riser bar idea and decided to just get used to the flat bar instead.
Unfortunately, the chain tensioner was causing problems (jumping off under load) as it wasn't allowing enough chain to wrap around the rear cog.
So I bought a DMR STS tensioner to replace it. It doesn't look as nice, but it pushes the chain up rather than down so the chain is in contact with more teeth on the cog.
I also had to use a half link to get the chain as short as possible, but as you can see from the photos, it was still very slack and the tensioner has had to push the chain up right behind the stay to get enough tension.
But now it works perfectly and my first ride around the Kitcheners trail at Sherwood Pines was good fun (if a bit painfull and tiring with no suspension and one gear!)