Try using two (or three) of the same copper washers under the plug sealing washer. If that works store some extras with your spare spark plugs.
may be worth trying. What i tried was one of theseflat copper washers
http://www.ebay.co.uk/itm/14-mm-cop...e=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649
it was too thin.
I can find 1.5mm and 2mm flat copper sealing washers, like this http://www.ebay.co.uk/itm/10pcs-20m...e=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649
or in aluminium http://www.ebay.co.uk/itm/Aluminium...var=570158218423&_trksid=p2055119.m1438.l2649
or I can find the toroidal/tube crush washers like this http://www.ebay.co.uk/itm/10-x-14MM...27-/270974594214?_trksid=p2141725.m3641.l6368 and this http://www.ebay.co.uk/itm/14mm-Copp...pk-/291560923350?_trksid=p2141725.m3641.l6368
Those may be too large on the od to fit in the counterbore though, which is annoying as they'd have much more squishiness and ability to seal my mess.
I originally went for the thin washers as i didn't want to have the plug not reach down to the chamber so far, but needs must and a seal is important, hmm I might try the flat 2mm washers. I wonder about puling the sealing washer on the plug off, using the thick flt washer and a smear of gasket compound. various things to try, just got to decide which to try first and buy some more washers.