Left side of the boot, but you might have problems with the carpet...which can be solved with a Stanley knife. Or back of the seats. Or, if you have an MDF shelf, under that.
My mechanic fitted ours to the inside of the rear panel - NOT RECOMMENDED! Will be getting him to fit it elsewhere. As previous, the cut out is on the left side but don't know how 'less bouncy' that will be over the wheel arch?