Either a STURDY table or a counter like ic-racer.
NOT the fridge...vibrations.
Do NOT go low to the floor, your back will KILL you.
Think about your body position when working the enlarger. The lowest I would go is about 9 inches higher than the height of a step stool that I could sit on while working the enlarger. Even that would be uncomfortable.
I got an Omega D5 with the shorter standard girder than the XL girder for that reason. My darkroom is height limited and an XL chassis would put the baseboard down near the floor, which is NOT acceptable for me.