since you have limited space
it may not be possible
but a catalytic converter
can certainly be used like a 'resonator'
and will kill off noise in a
certain rpm range
depending on the inches of length
from engine exhaust port to where the
catalytic converter greatly expands inside

there is also a noise effect
that depends on the distance
from catalytic converter outlet
to exhaust tip
... here it is best to avoid anything between 55 and 75 inches if you don't want
'highway cruise drone'