I have a large 4' Puffer purchased many years ago form Martins Models at an E Port show. Originally I tried lead but it became very heavy to carry. I modified mine by making three large sealed fibreglass containers in side the hull which were open at the base and had open portholes at the top exiting thro the hull sides. I now only need about 1lb of lead plus the 12v 12Ahr battery to ballast. Put the model in the water then add the battery and ballast, the boat sinks to the waterline. Take the battery and ballast out and the boat rises, lift with two straps and all the water drains leaving a light hull to carry. The pic shows the model in its lead ballast days.

Suggest try a mixture of old car wheel weights. The radial ones are usually good where they can be slide in and for more congested spaces use the square stick on ones.
Most tyre companies are only too pleased to get rid of them. Leave them to soak in paint stripper, wash well in cold water and most of the old paint and adhesive will come off. The remainder can be got off with mechanical endeavours.
Once located, saturate in glass fibre resin and they are fastened in for life. if they need to be removable use double sided tape.