After mulling it over for several years, and finally getting tired of all of the different saddles slipping forward, I finally gave in and bought Tonto a crupper.
Tonto said, "I don't like the sound of that."
"It's gonna go where?"

Yup!
So we fitted it...

Then tried it out on the lunge...

"You said go?"

sigh... "Okay...."

"It isn't
too bad."

There was no rodeo, and Tonto seemed to accept it just fine.
I don't think Casey Cat liked the idea of it though:

And Scout was VERY thankful he didn't need a crupper...

So, after I lunged Tonto with the crupper in both directions for a good bit, like a good Mom, I put my daughter on him (and led them around). Tonto was fine with it, so after dragging them around the pasture a few times, up and down the hills, I climbed on. It seemed to do the trick, at least at the walk...I was too chicken to try anything faster until we have a few more goes with it. Hopefully, it will solve our saddle slipping problems and we won't ride up Tonto's neck any longer.