When I was doing this, my solution was two sheets of glass, hinged at the back with black masking tape. Then one strip of tape to position the back of back neg strip, and one along the front of the front strip. This was all arranged to fit in the center of the enlarger's view when the glass was pushed back against my D2Vs carrier-locating pins.
Lay the negs in place, and with the tape to keep them from wandering, as they flatten they line themselves up as you close the glass. The glass was long enough to hold three strips of six, printing the first three negs, then sliding the carrier along to print the second three.
Total cost: about nothing, and ten minutes of my time.