I would suggest going to the Quadratec site and taking a look at the install directions for the Best Top Flexa Trunk, the Best Top Instatrunk and the Tuffy Security Deck with Enclosure for suggestions.
Here's a link to the diections for the Security Deck and Enclosure, which seems to have the most robust mounting:
http://www.quadratec.com/Assets/Installation/114832.pdf
I have thought of doing something similar, but don't want to give up the under floor cubby. I figured a raised floor high enough to partially open the cubby hatch to allow access would provide for sufficient storage up top. So the basic structure I have had in mind would be H shaped with a back, with the mounts to the tub sides and to the seat mounting studs, like the Tuffy Security Deck Enclosure. If the right to left span is too great for the weight, I would put in a support "column" under the raised floor, forward of the cubby hatch hinge.
To mount to the seat studs you would need to temporarily remove the seat, but only to mount the structure. Plan B would be to mount to the sides only...
JPK