I build my hives with a cleat in front for spacing at front, but the back of the hive is clear for top bars to slide off the back if they wish. Top bars swelling while in a tight space like that is guaranteed to split your hive.
Wood blocks were used in ancient times to split granite blocks into smaller pieces by jamming into cracks and swelling with water.
You are better off leaving out the last topbar altogether until you can replace it with something smaller.
Next time you make a hive, don't enclose the topbars on both ends. Leave the back open so that the hive is still closed even if the last bar overlaps the end.
Scot Mc Pherson<br />McPherson Family Honey Farms<br />Davenport, IA<br />BeeWiki: <a href=\"http://beewiki.linuxfromscratch.org\" target=\"_blank\">http://beewiki.linuxfromscratch.org</a> <br /><br />Pics:<br /> <a href=\"http://linuxfromscratch.org/~scot/pics/bees/\" target=\"_blank\">http://linuxfromscratch.org/~scot/pics/bees/</a>