thats alot of work to get some vibrations under your ass. why not jsut get a vibrator and wedge it in the space between the backrest and the seat bottom
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in all seriousness and despensing with the rest of my thoughts about this question...it can be done i guess.
id think somthing small like an 8" would be fine. youd likely have to build a box but thats not real hard...jsut make sure you look for the compact woffers(think thats what they are called) or somthign as narrow as possible.
ON and you HAVE to put on under each seat if your doin this. im sure any lady friends along for the ride would...enjoy it
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