The solution below is by Michael Keller of Baltimore, forming all 369 octominoes into a single 51x58 rectangle. Notice his strategy for solving this monster: he used the complicated pieces first and saved the easy, more cooperative shapes to the last, where they are clustered in the lower left quadrant. The six unfillable holes are dotted with a contrasting color. Michael is one of the world's top polyform puzzlists and once self-published, for 16 years, an excellent magazine, WGR. WGR stood for World Game Review, but through a labyrinth of legalities he ended up losing the right to call it that and gave up publishing it altogether. The superb editorial contents may eventually be preserved online.
This is the solution that fit into the original (1989) custom-built wooden showcase with scratch-resistant clear window. It was magnificent. Sorry, no photo of it exists. That was in the days before digital.