Poundland have some large baubles at present that could be useful, at least 5 different styles, including one that has a sort of quilted appearance and looks ideal for a balloon. About 6 inch diameter. Of the other styles, one has a spike so looks like a minaret if you cut it down with a hot-wire cutter, would be handy for scenery. A couple of the others might make handy SF pods or perhaps Martian habitation domes? On the plus side, most years they drop all their Christmas prices by 50% just before Christmas