I have had good success using Evercoat Rage sanding body filler. I thoroughly clean the cap surface along with the well and underside of brake plates, then line the whole area with aluminum foil, then close and lock the spoilers. If your spoilers don't lock inside the wing, you will need to come up with a method to apply enough closing pressure to make the spoiler caps seat all around (believe we used a wooden stick that jammed against the root rib to keep them locked on a LS-3a) mix up a good batch of Rage and spread it evenly into the well........may take several applications to get everything high. Then cross sand with an 18" long board and 40 grit paper, 45 left then 45 right strokes till you start cutting into the aluminum foil. At this point you can fine fill with Evercoat finishing and blending putty followed by heavy primer, etc.
Feel free to email me with any specific issues,
JJ