For really large prints -- crude but cheap:
1. Large sheet of non-stainless steel. Strips of magnetic tape. Quick and dirty, but not very cheap.
2. Large flat sheet of fiberboard or plywood and straight strips of opaque cardboard such as thin matte board. You can glue up two long 'L' strips of different widths of the thin opaque cardboard [ ------- ] and then glue them to the ply at right angles to each other. The lip ^ overhang forms the border. Slip the paper under the channel lips to position it. Registration nails will then permit you to position two plain strips to form the other two borders. Additional registration nails permit size adjustment. You are now limited only by the size of the paper. Takes time to construct [paint the board, etc.], but it's almost as cheap as dirt.
Additionally, you might have to turn the enlarger column 180 degrees on its baseboard so that you can project onto the floor. Remember to weight the baseboard!
Finally,for really big prints, have a radio in the darkroom to provide entertainment while making those rather long exposures!