I recently got to try out a BassPac backpack-based LF Bass driver rig.
Designed for gamers and anyone who needs to FEEL their bass, it was effective and reminded me of those car systems from college with 15" bass drivers where your internal organs start vibrating!
When I checked the price later though (these things go for around USD$300!) I started thinking about hacking something together.
Decided that the actual backpack part would be easy - a regular hikers water carrier will be the 'case' of choice:
It already has a pocket which will be about the right size and will hold the unit close to the wearer's back.
I also have a redundant PC speaker set with subwoofer which will form the basic electronics.
Still need to arrange a suitable battery for this.
I plan to test with the original speaker (although it's only a cheapie) as a POC. Researching around the Dayton Audio Bass Exciter looks like it will operate down the the frequency range required (sub 50Hz), so I will source one of these eventually.