I dont know for sure if the splines on both sides of
the spool line up or not but a end of a axle(splined
end) would work.... if its 1.5" like said then cut 1.5"
off of the axle spline and stick it in there, you
might be better off having the spacer shorter and have
all the spline you can get into the spool
EDIT
you never did say what kind of racing you are doing
lets hear it.... is this drifting