i've never had a manic mechanic adapter slip. i run them on my race bikes, and bikes that have gone over 50mph, with lots of torque.
in fact, i've never re-tightened the one on my 24" worksman wheel with shimano coaster, that was put on over 2 years ago, and has been on numerous bikes.
i've used them on at least 5 builds with no problems except for a sprocket bolt backing out on one of them.
i have a Sportsman Flyer adapter i used for a short time, only because the smallest sprocket is a 34t, and i usually run a 30, and have had no issues with it slipping either. it's a 3 part adapter, unlike the manic which has 2 parts.
i've never had a Howard adapter, and it looks like a quality piece, so i have no input on it.
just wanted to make a statement from my experiences to dispel claims of "slipping."
if it's slipping, it's either the wrong size, or not tightened down correctly.
there are very few HT's out there being ridden as hard as mine, or as fast and torque-y as mine, so i feel confident about my knowledge on this subject, and feel i'm able to give an unbiased opinion.
i'm not knocking anyone's product, just the claims made about them.
none of this really matters to the OP if he can't find what he wants, though.