Help With Fusion360

Hello,

Can anyone recommend any strategies for designing this gear cover? I need to customize it for a particular size pinion gear and hex head socket cap screws.

I have already tried combining bodies but subtracting was not working

I also tried starting with a 2 D design but then I cannot shell the design.

Here is the item with two conical shaped designs: https://www.amainhobbies.com/rpm-ecx-torment-ruckus-circuit-gear-cover-black-rpm70672/p438207?gclid=EAIaIQobChMIxuuo_r2I5wIVh5WzCh2wjADZEAQYAiABEgLNhfD_BwE

Thank you in advance