I've got the axio on an '07, not sure if the '08 tails are smaller or larger.
I didn't use the velcro straps because the rear one covers up the tail light so I ended up using cable ties to strap it down instead. Its just slightly bigger than the rear seat but it fits on there pretty well.
i'm not a big fan of bags. if it doesn't fit under the rear set or in jacket pockets it doesn't come with me (my wife doesn't ride and she is ushally going were i'm going so i have a suv as my bag!). however i have used a back pack a few dozen times before. but if you need you need it, it does the job.