A buddy had a box mounted in his pickup bed -- He drilled through the bed, put long bolts from the bottom up, one nut on top of the pickup bed, then fed the bolts through the box, then had large wing nuts holding the box in place from the inside. No brackets for anyone to be able to remove from outside, and no bolts they could get at unless they were already able to get inside the box.
Horrible ascii art:
W W
WWW - Wingnut
------------------------------------ - Bottom of box
=== - Nut
-------------X------------------- - Pickup Bed
XXX - Bolt head
-Al.