These use AR buffer weights which come in aluminum, steel, or tungsten varieties (as well as a printed option for fine tuning weights). I recommend the steel weights because they are cheap and decently heavy. At the time of writing this they cost $1 from KAK.

They’re cheap, get a bunch.

I recommend you start with a +4 option, as the heavier options may not run with your setup. You could of course print a few different ones and play around.

The deadblow options are mostly just silly, but I left them in there if you want to play with them too.