Try this one at Freescale.
I haven't used it but it's product specific. Find a similar FET. The data should be more reliable.
MTTF calculator available at http://www.freescale.com/rf. Select Tools/Software/Application Software/Calculators to access the MTTF calculators by product
I'll see what method our LPKF guy uses.
We regularly cut slots in test fixture components we make from board material.
I'll try to get back tomorrow and post the method.
http://www.eccosorb.com/
http://www.rfcafe.com/vendors/components/magnet_shield_links.htm
Try these sites for materials for your enclosure that may help.