get some folders and label them top secret....put some hidden tamper seals on them.

get a large sum of stage/fake money and put visible tamper seals on it...in your checked luggage.

heck put tamper seals on everything! Would be nice to see what they open.
http://www.americancasting.com/info-...ity-01-TOC.asp

put ink/dye in your previously mentioned envelopes

put a running audio recorder in your checked luggage