Canoe and Kayak Tie Down Anchor Locations? - Gen 4

We wish to carry a 16-17ft canoe on our Gen 4 and desire suggestions for where to anchor front and rear tie downs at the right and left. The two towing eye anchors at the front and single eye anchor at the right rear appear to be candidates.

1. Can anyone suggest a specific, reasonably priced alternative anchor eye or bolt to fit the car (Toyota's price for a towing eye is beyond belief). Is the anchor's thread specification known (I can measure the pitch, but which standard is the thread)?

2. Are there suggestions for a rear left anchor point? Both the lower control arm and the body anchor stud (or bolt) to the rear subframe appear serviceable but require sliding under the car and we'd prefer a place that can be reached without having to lie on one's back. Are there good places (existing bolts or holes in sound metal) under the bumper and underbody plastic covers (I'm willing to cut access holes)?

When carried on other cars our boat occasionally gets hammered by cross gusts stronger than the front and rear stays (and straps at the carrier bars) can control, twisting the boat sideways and vastly increasing the wind load - we've had success tying another stay (rope, or nylon webbing for more tire clearance) to the front coil spring on the windward side, padding it with foam pipe insulation and, when we care about the car's finish, applying clear body tape to that area of the fender.

 

Thanks. I prefer to tie off to the frame or something solid that is connected to it, especially at the front, for several reasons: Driving across the planes at interstate speeds while passing semi's in heavy cross winds, gusts can really "rock the boat" in ways that I can imagine damaging thin sheet metal. I've encountered similar gusts off the ocean while approaching the GG Bridge from Marin on windy afternoons, and have had anchor straps abrade paint on other cars.

One solution for the side loads from cross gusts is anchoring to the front coil springs using a loops of webbing, padded with soft fabric where they wrap around the fender edges - that hasn't hurt my other cars (Merc. Villager, Volvo, Outback) but I haven't tried it with the Prius.