[alanw]

Is the extender so high that it keeps the main portion of the yak off the bed of the truck? Are you strapping it down real tight? You can try strapping it from the back of the kayak to the extender instead of over the top of the yak. I think the best way is probably the saddle setup as mentioned. I haul my kayak without a bed extender with no problems, and I usually have some gear in it so I'd rather not transport it upside down.

For an assist strap I would attach it to the front handle attachment points.. they should be plenty strong enough.