I agree with Andy.
I normally use lead shot fishing weights, which are small enough to fit into awkward areas. These are held in place with children's 'Plasticine' (synthetic modelling 'play-clay'), which can be moulded around the weights, holding them firmly in place. Apart from providing extra weight, the clay will not damage the plastic, unlike some glues, which can cause distortion.
At the moment, I'm out of the lead shot, so I'm using small strips of roofing lead. Off-cuts can often be had from builders merchants, and the small piece I have will probably last me years !