smother the top of the wall with grease. proper machine grease that they grease trucks with, the 5th wheel and propshaft etc. it comes in tubes about the size of a roll of black bags. that stuff NEVER comes out of clothes. and its plenty thick enough to stay on the wall forever really.
the tubes look like this,

http://www.acehardware.com/product/i...entPage=family

but thats a tad expensive for your needs

and a shotgut cartridge trip wire would shit anyone up i'd imagine

http://www.defensedevices.com/shotgu...m-signal1.html