A motor box is a much more secure method of attaching an engine. The standoffs create a cantilever effect which is susceptible to vibration. The longer the standoffs, the more the vibration. You will notice it most at idle. Make the box with 3/8 inch aircraft plywood with plenty of hard wood triangular stock and 3 hour epoxy. If you do not have room for all of this, then you are stuck with the standoffs. Dan.