Castoffs rhymes with:
cutoffs, cutoffs, drop-offs, dropoffs, hoelterhoff's, knockoffs, layoffs, payoffs, playoffs, ripoffs, selloffs, spinoffs, standoffs, takeoffs, tarnoff's, tartikoff's, tartikoffs, tradeoffs
Castoffs sounds like:
catfish, cdebaca, codifies, cuddeback, cutback, cutbacks, cutoffs