The 1.5" on the back of my box doesn't cover the entire back. Its the size of my amp plus about 4" all the way around. Like a mounting board just for the amp, I carpeted it front and back then bolted it very tight to the box, i figured both the extra thickness and carpet would help isolate the vibrationsthe box is well braced i just didnt want to bolt it straight to it. if i have enough wood ill do a second back side