Body filler for Fiberglass?

Can anyone suggest a body filler that is recommended for direct application to fiberglass?

I'm finishing up modifications to a truck cap and am ready to start the bodywork.

Quick backstory:
I recently acquired a '17 Tacoma Double Cab short bed; and have a road trip planned for this summer. Although I don't want a cap for the truck, having one for this trip would be a big benefit.

I'm unwilling to spend $2000 for a new cap, and have not found any used ones for the 3rd generation Tacoma, so I got an old $50 Leer cap that fits a 6' bed, '00 S-10 and have shortened and widened it to fit the Tacoma.

I shortened it by 13.5" and widened it at the front by 2" and at the rear by 1.5". I removed the side windows and glassed over the openings. It's ready for bodywork and paint.

 

I use this stuff every day, I prefer the 3m platinum plus or evercoat z-grip. Both are fine going over fiex-prime.

I'd probably start with 180 under the first coat and finish in 180 or 320 before primer. It will help eliminate sand scratches showing through and causing a re-prime. Just keep watch for pinholes, they show up big time after paint.