i reinforce fabric made pincushions with woven iron-on interfacing. fill base w walnut shells for weight, top off with polyfil. if for special friend i also add a "strawberry" at top filled with emery powder for needles. be sure that you check with potential recipients re: nut allergy. if a problem, try fish tank sand.
besides the stopping porosity of fabric weave allowing migration of walnut shells, sand or whatever granular filler you use, the interfacing helps re-inforce seams if tightly stuffed with polyfil.
also ...for any pincushion that needs to stay upright, i place one or two round (or whatever shape of base) circles of light cardboard before adding shells. i use the cardboard of cereal boxes, backs of tablets ...whatever have at hand.