Potholes aren't nautical! (well, apart form the ones big enough to fill with rain & go ribbing on.....
I don't bopther with a ratchet at the front as the winch holds it down, but I do however have a 10mm wire that clips on to the bow eye and heads aft to the chassis (in case of a shunt - should stop or slow the boat on it's momentum induced attempt to keep going) it goes tight as the boat hits the snubber. There is a 6mm one that is slack, but should the winch and both aft ratchets fail stop it rolling off the back. (but by the time that happens I;d like to think I;d have noticed a flapping web or two in the mirrors) that one also doubles as a "boat on the slip" emergency cord.
As for holding it down, if the winch lets go, I'll notice the vertical movement in the mirrorr - at night the bow is lit by the car's tail lamps.
Over the toobs? No way - all I need is the sun to come up or go down & the toobs harden / soften. Fix to the solid bits!
Beamishken - I assume you have a couple of bolts holding your engine on? Why not replace them with eyebolts?