I use regular straps and then loop a bike security cable through the scupper/handle on the kayaks then lock it to the roof rack with a U lock - Both of which i already had on hand. Like u/fudpuck3r says, this (and locking straps) really only deters casual theft/dumbassery. anyone who really had the means and intent to steal the boat will have a lock cutter. I dont think that super likely, it just gives me a little peace of mind when we are out and about after a paddle. keep honest people honest.
Master Lock 8170D U-Lock Bike Lock with Key, 1 Pack , 6-1/8 in. Wide , Black https://www.amazon.com/dp/B0017L4PAC/ref=cm_sw_r_cp_api_glt_fabc_G2KQBXZWMVY6A8WYX5AZ
one like this one
I had one like this one and they were still able to break it and steal my bike
Deal link: Amazon
^^Note: ^^The ^^deal ^^may ^^have ^^expired ^^by ^^the ^^time ^^you ^^see ^^this ^^post.
For $7 a fun upgrade secured to their side mirror
Master Lock 8170D Fusion U-Lock, 6-1/8 x 8-inch https://www.amazon.com/dp/B0017L4PAC/ref=cm_sw_r_cp_api_i_lodiCbKFTSEJX