I kept my TI in the water at the Lower Murray Lakes for 10 days. Had a S/steel cable made up at the hardware with an eye each end and padlocked to the Aka inside the fitting and to a post on the bank. I have also used the cable to secure to a tree stump. I removed the paddles (easy to pinch) and the mirage drives (hard to replace due to $) so there wasn't a lot to carry.
I know this was no guarantee if someone was determined but it should stop the joy riders.
Not sure what "anchor" your other craft means but bite the bullet and buy a Hobie Kayak. I have had 28' craft down to small catermerans and have had more fun per dollar from the TI.
Cheers, Brian in South Australia
Tandem Island -