ABUS Bordo Granit 6500K Folding Lock with SH Lock Holder - Bike Lock with XPlus Cylinder for high Tamper Protection - Security Level 15 - Length 90 cm | bike-locks

a folding lock like this ABUS Bordo Granit would strap on very nicely to the seat tube. it's compact, and wouldn't rattle and make noise like a chain wrapped around the seat tube would.