Just make a cavity in your print. Pause before it closes up. Drop the weight in. Penny or nuts. Also I pause and put in some glue when I drop it in so it doesn't roll around inside.

You could also make cutouts for pennies or nuts on the bottom and glue them in after the print finishes.

There is a Lego phone holder that does the cavity.

https://makerworld.com/models/660273

Hope that helps.

I made this. Used nuts cause I didn't have any pennies. Worked pretty well.

There are pictures in the description. So check those out.