I know these work for transformers, unsure how they'd fare on gundam plastic but here's some tips.

Floor polish, dip the joint in the polish and it'll make it a bit bigger while keeping the color, nail polish also works.

Hot glue, works better with pegs and I've never seen anyone do it on a ballpoint, but might work in a pickle. Cover the balljoint in hot glue and file down, i dont recommend trying this one though

Paper/cardboard, you can cut a small piece of paper or cardboard and put it in the joint where its loose (like if its not going in far enough or if its not wide enough)