You should be in 2 rail, and you should sell those to Lionel people everywhere.
I'll do you one better. Here's a link to the STL files for it. Anyone can download and print their own.
It's printed in 4 parts and then glued together. If your RS-3 is a Lionel from the past 20 years or so, then the frame is molded plastic and the undercarriage is molded into it. This is sized perfectly to fit over and cover the existing undercarriage tanks. (By the way, if anyone knows better terminology or what exactly all these parts are, I would love to learn more about it.)
If your Lionel RS-3 is from the 80s or 90s, those had a metal frame and the undercarriage was a plastic part screwed on. You can take it off and replace with this.