Gas cap for my reserve tank.

[68GMC2500]

Hi all,
I'm having trouble trying to find a replacement gas cap for my reserve gas tank.
From what I've gathered the tank was added on there's a switch by the seat to switch the tank but the opening is smaller than the factory tank.
Parts guys don't have a clue and they're not willing to help. Somebody stole my cap and now it's been a pain in the butt to replace it.
Truck is a 71 GMC 2500 long bed.
Thanks for any help.