Call me chicken-- I have yet to see a portable restraint which I would trust.
I still the guys who rope their plane to a fence - take a big chicken stick and slap the prop with engine at high idle - the plane lunges forward at the restraint -
somehow the rope holds -once more -
The shock load could easily rip the rope /fence/stabs .
buried posts which are 3 ft high and padded can be used as a restraint for the wings- possibly the safest "stooge
still , a helper who knows what is going on - holding the plane is the best
At least if you really screw up -someone is there to take you to the Emergency Room--
I sometimes use a single post against the left wing and me close to the other wing- should something go wild the plane rotates the one wing against me. This OK for small planes but anything over 40cc no thanks - I get a helper or don't fly.
We MUST use an observer at our field anyway regulations because of light aircraft traffic - many of the old farts in em can't see a 600 ft paved runway from 1000 ft -- or 50 ft for that matter .