Haven't tried this, it should work very well - Use some rare-earth magnets
Suppose you get yourself:
4 or 5 small rubber well nuts (these go in the factory holes in your trunk lid)
http://www.hansonrivet.com/w71.htm
4 or 5 flat head machine screws with matching thread of your well nuts
4 or 5 of these or similar roundbase magnets
(95lb pull variety should be more than plenty magnetic goodness)
http://www.magneticdepot.com/roundbase.htm
drill out hole as needed for flat head machine screw in one half of the roundbase
the rubber well nuts go into the 4 or 5 factory holes in the trunk lid
screw one half of the roundbase with the magnet in it into the well nut, the rubber will expand nicely and the magnet and roundbase will not pull out of the factory holes in your trunk (assuming you have appropriate diameter well nuts)
make a stencil to attach your other half roundbases to the back of the undercover
Use screws, epoxy, etc to attach other half of roundbase to the back of your undercover, make sure these puppies are attached VERY well in case you want to remove your undercover someday and not have this half of the roundbase tear away and stay with the magnet side in the trunk
Mount the undercover and enjoy, the magnets should do the rest for you for a very long time, regardless of heat, moisture, bumps, etc
...and if you want, velcro the crap out of it too... : )