QR on the bottom and wedged at the top.

Thanks for pointing that out... I normally have a velcro strap around the seat stay by the brake (left off while cleaning in our "off-season"). Probably not necessary, but extra insurance. There is a significant "dimple" that the QR lever fits into, and it's a two-handed effort to set the pump in place.

I've had pumps eject when hitting a bump while mounted vertically in front of the seat tube. I've never tried the horizontal mount under the top tube even though several of my bikes have pump pegs.