...For weights i went to the fishing tackle box and got some slit "shot like" sinkers , drilled out base and banged them in. They come in a variety of sizes so you can add weight incrementally in small amounts.
On one of my Pinewood cars when I was a kid, my dad dug out his old lead melting setup from when he used to cast bullets, and poured molten lead into holes drilled into the wood. As I recall it got kind of smoky...I just know we sought a different approach the next year.
Ideally it would be someone with plenty of room. And perhaps a large hill in their back yard. With lots of dry weather. And video recording gear. And beer.
Sounds like a summer gathering needs to be planned. Hint, hint, hint, somebody. Maybe somebody in the Midwest. I'd make plane reservations but I don't know where to or when.