While shopping at the arcade section of Lowes home improvement warehouse, I found what I'm going to use for marquee retainer for
Switchcade.
It's a steel track designed to hang closet shelving systems from. Comes in white powder coat but I'm going sand it and paint to match the cab. Basically very similar to the Happ's plastic retainer but locally available in a lot of markets.
In the second pic the end is bent, but I'll cut that off.