You can DIY a carrier
: have a piece of single strength glass cut to the full size of your negative stage. Then have a thin piece of plexy cut to the same size. Tape one edge of the two pieces together to make a hinged sandwich. Finally, cut a piece of black construction paper to the same size as the glass/plexy sandwich and cut an opening in the construction paper to the appropriate size for your negative. You now have a negative carrier and it's cheap. It has the advantage of keeping the negative very flat--no popping. Just make sure you keep all surfaces clean.