You can just use straight spot putty and back it up with a high quality duct tape to make this fillet.
Use some boxboard with wax paper taped to the spoiler as a form, apply spot putty and put a precut piece of duct tape and sink it into the wet putty.
Just don't make the casting too thick on the back.
Have the tape go past the fix area onto sanded solvent washed urethane.
Remove waxed boxboard and finish fillet with sandpaper.
The fillet won't crack or fall out unless you knock the spoiler off somehow.
I hit an IBeam on the shop floor someone left too far out into the drive path one dark sat morning and the fillet broke when the spoiler got knocked off.
I need to repaint the spoiler due to some scratches
from my lift so I'll redo it then.
Finally sorted out the Optispark on my SC 383 97 Camaro M6 Vert so I'll get to this soon.
Somehow this Golen 383 race engine would grind up Optis after a few weeks.
The rotor in the AC Delco opti if pushed like this will shatter and the car will die.
At least the MSD cap and rotor will live long enough misfiring under load to get to the shop and up on the lift.
The cam pin that drives the Opti at the front of the engine had no clearance but didn't really seem too long so I put the Opti in 1 washer thickness out which didn't compromise the oil seal and now it lives!
Getting to the cam retainer bearing to check it requires dropping the pan to button it back up after removing the timing cover.
Yikes!
Golen said the cam would fall out without the retainer bearing.
For some inexplicable reason the cam was slightly pushing on the distributor.
What a headscratching pain that was to figure out and fix after a going through a few optis.
Opti changes are a pain.
Also you could just sand the cut area down to a smooth taper which is ok (see pic)but I did it this way for a nicer tighter fit.