or stand closer
To be honest I'd think twice about buying a TC to use with slow zooms. if i assume you want more reach, then you're most likely to be at the long end of the zooms, so your 5.6 aperture becomes f/11 with a x2 TC.
What Dave said. The reason your jessops convertor didn't work was likely because the combination of lenses produced an effective aperture too slow for the AF system to work.
Even the under 12's in the WPOTY competition pack several grands' worth of glass.
Or crop the image?
It confused the hell out of the staff in the shop
If its a one-off you could possibly consider hiring the equipment