Gday nathan
You could try sealing the crack with a dab of nail polish .but make sure its dry before putting it back under hen.
unless egg is 18 days old which means the lil fella mite be on his way out
goodluck
gday all
we have another tassie boy on line . click on link to watch my budgies shake a tail feather http://tim-philpott.blogspot.com/2010/05/budgie-shake-your-tailfeather.html
This was made by my brother inlaw .he has done a great job my kids and i love it hope you al do to .fill free to pass it on
cheers tazman