Thats just not gonna work...risers arent really a solution ...
A buddy has size 13 rulers which are "shrunk" down a size to be more like 12's..and he has minor overhang on a 25.9cm board. With size 15 you need a full wide like 27cm
'09 151 Never Summer SL-R
'10 K2 T1 DB Bots
'09 Burton Cartel Bindings