That instructable is for a tiny and lame vacuum former.
It's easy to make a much better one.
http://www.tk560.com/phpBB2/viewtopic.php?t=466
http://www.rcgroups.com/forums/showthread.php?t=621858
http://www.tk560.com/phpBB2/viewtopic.php?t=502
The vacuum forming forum on the tk560 discussion board is where it's at:
http://www.tk560.com/phpBB2/viewforum.php?f=1
BTW, it'd probably have been better to have bumped this earlier thread on the topic here on TDH:
http://www.thedentedhelmet.com/showthread.php?t=17010
Followups should probably go there so stuff doesn't end up scattered everywhere.