Leave cavities for steel things like big bolts, shaped so the bolts won’t rattle around. Add a pause to the print before covering the cavities. Disable support for the cavities.

When the printer stops, stuff the cavities with bolts, then resume the print.

I’ve been a fan of making false bottom pieces and using steel wheel weights in them.

Here’s a lamp I made as an example.

I used wheel balancing weights (tiny 5g iron bars) and built them into my prints.

They are pretty cheap and the included adhesive makes it so they don’t rattle around inside if the tolerances of the cavity are not perfect.

Used them in the base of a tall slim model to make it less prone to tipping over.

I don’t know a general approach, but in one particular print with low infill I paused the print to pour in some sand just to make it more bottom heavy. While it worked great in that case, I don’t know how well it’d work in other circumstances. Great cost/weight ratio, though.