You are spot on with regard to buying used gear as a money-saving strategy. Our OP seemed to indicate a wish for a DIY experience. Planning to use harvested down from thrift shop items might work, but the down is likely to be of lesser quality, either beaten to near end-of-usefulness or mixed with a lot of feathers. Coupled with the effort to make a down item, it may not be worth it. Weight was not an issue, making synthetic fills good options.