I put together a low wing trainer for someone a few months ago, that needed a LARGE amount of weight in the nose.

This was because the engine I selected for the newbie was purposely a bit small.

To add the weight I put a blind nut in the engine compartment, then put a long lag screw through a bunch of fender washers.
A small stack of them adds quite a bit of weight.

I was quite pleased with this arrangement as I could easily add or remove additional washers as required, or pull the whole thing off easily if I changed the engine.

Very easy to do, and there is no way the weights will come off.