I have plans for modifying aluminum blast gates to auto-closing for about $12.00 for each gate:
http://cgi.ebay.com/Automatic-Dust-Extractor-Blast-Gate-Plans_W0QQitemZ110428547687QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item19b60dea67&_trksid=p3286.c0.m14