i have this speaker.... http://www.cobra.com/shop_image/prod...e1a73757e6.jpg
i took the mounting bracket off, pulled the four screws in each corner that hold the front and back together, replaced those with 'eye' screws that were long enough to hold the two halves together and have the 'eye' flush with the box. then took two long zipties and ran each through the two 'eyes' on each side of the speaker and then ran those around the rollbar right above and to the left of the driver's seat. tightened them and speaker secured to the rollbar.
i kinda like the idea of flushmounting it in the soundbar though. would be mostly hidden when the top is off, protected from rain, and in a great spot to effectively hear it.