you can make a print ( with your enlarger ), the size you want your final alt process print to be ...
and contact print it onto a sheet of fiber paper. then WAX the paper and use that as your alternative process negative.
the "internegative" can be hand coated paper if you want, or a sheet of xray film or lith film or even
a sheet of pan film ... just make test strips and make your print ... you could also make a xerox
of the negative and wax that ...