Hi.
Does your insert have a bottom to it or is it cut away?
I have seen people just sticking a slab of roofing lead underneath thier seat insert before, but it makes you sit higher in the kart.
The standard way is to duct-tape or cable-tie the lead flashing to the outside of the insert.
Alternativley, you can cast yourself some weights in tuna tins (no joke) and use them on the pegs provided. They are rated to a max of 12kg i recall.
I race heavyweights, so have had no personal experience of ballasting up...
Enjoy!