Im in the bandwagon that the only speakers this needs is the pipes on the side
However, there are a number of places where speakers could be fitted to a M3W. I agree with greenteagod that something could be fabricated to sit on under the dash in front/on top of the transmission tunnel.
Personally, unless you are going for very long drives, I would just put some velcro on a portable bluetooth speaker (boombox). Beats and Bose make variants. Most have at least 7 hours or battery life with constant playing at moderate bass.
