Jan,
Whatever works for you is what you should use. My original post was in specific reference to plans that Bmac had posted and his reference to those plans being suitable for ULF. In reponse to those plans and for the benefit of sharing experience garnered by others and myself, I shared what my experience was.

I also shoot 12X20 and personally have opted to use a vacuum frame (albeit of smaller size then what you related). A 30X40 vacuum frame is almost four times as large as is needed for a 12X20 negative.

A number of people that visit this site shoot ULF of various sizes. Those that I have knowledge of seem to indicate that they use vacuum frames (the only exceptions that I can recall are David Goldfarb (11X14) and yourself.

Is the frame that you constructed also of a 30X40 inch size?