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| A 15x15 hexomino pattern would have fit nicely into that central hole; I wish I'd thought of that before I'd finished drawing up the image because it'd probably look nice, but I can't be bothered to change it now. |
I also found a four-colouring for it too, which took a little while and a little bit of trial and error. No two pieces sharing an edge are the same colour, and when four pieces meet at a point none are the same colour there either. (At least, I hope so. There's a chance I've slipped up somewhere and missed it.)
Full solve time must have been close to six hours, with the repeated backtracking phase of putting the last 15 or 20 pieces in taking unreasonably long for whatever reason. About two hours on the 22nd, and another half hour on the 23rd until I got that little 'eureka' moment and the last few bits went in perfectly.
Edit:
I found a middle bit for it anyway - the 289 square central hole fits the pentominoes and hexominoes with 19 squares to spare, which doesn't allow for any particularly nice configurations. The picture below is as good as I could get; sadly it doesn't preserve the overall symmetry of the octominoes' outer shape.
