It seems like a lot of work the first time that you do it, if I had to do it again it would take about a quarter of the time. I had never done this before so I went slow and it took most of the day to do. The new adapters from www.subthump.com
did not come with any instructions, but it looked easy enough to install.
you could use this adapter from the bottom or the top because the screw holes for the adapter will go into the sheetmetal. If you use this adapter to mount underneath it is 3/4 of inch thick, so your speakers will hang lower and may get in the way of things. If you dont care I don't see why you couldnt.