pictures would help. this is your basic blackburn clone?
adapter? you mean the long (usually) flat metal strut, usually with some
bends in it? bolts to bottom of rack, other end bolts to fork crown?
if so, and it don't fit....throw it in the parts bin for another day. just
about anything will work. i'm pretty sure the weight is carried by the
legs, the support strut/adaptor mainly holds the rack in place.
i hit the maintenance shop here at work, they got a couple hundred
wall-mount fans getting thrown own....pulled some metal support
strips, bent-cut-drilled one to fit.
and do use them self-locking nuts......