Nothing secret about it. Knowing that trunk space is a premium for the family etc I had good success with the setup I did.
Look at the rear right area behind the tire in the trunk. It's mostly a wasted space. Using cardboard to make a template I figured out dimensions and made a box using the glue etc. Once built, it fit in that space so snug that I never screwed it in or bolted it down. I went with a single sub that IIRC was a 12", might have been a 10" though. My setup was pretty simple and worked well in that it was all Boston Acoustics that bolted right in to factory sizes for front/rear speakers, amp was a 5 channel inverted in trunk with sub on rear right. Did a pioneer head unit with a backup camera and enjoyed it for years. On an amp, make sure if you consider bolting to the bottom of the rear deck that it's rated to be oriented that way.
