though you could really use either style cap you like

Customize Your Style: Easily switch up the look of your bike with a variety of filler caps sold separately

Filler accepts a Harley-Davidson style thread-in gas cap (not included)

Chrome plated reproduction fasteners to mount gas tank on 1937-1952 45 Solo and Servicar 1937-up

There are various options for custom battery boxes for motorcycles made specifically to hold AntiGravity batteries

#3218-1 Steering Damper Knob 1952-1966 Harley-Davidson K/KH/XL 1960-1984 FL/FLH 1960-Up Servi-car Vendor_Antigravity Batteries though you could really useReproduction of OEM# 46746 52 aluminum knob used on 1952 1966 Harley Davidson K, KH, Sportster (except 1959 CH), 1960 up Servi car and 1960 1984 FL, FLH. Made in Ohio, USA.