Years ago, my Brother and I made some prints this size. We made 'frames' with 2 X 4 lumber and lined them with plastic sheet from the building supply. This made big trays we left on the floor. Final wash was with a hose near the floor drain. This was all done in our basement at night. We also found that our enlarger didn't have enough light to do the job, so we used a slide projector with a 1000W bulb for exposure. Worked for us.