I've got a cart with a dead channel that I am probably going to remove, or try to remove from the plastic housing. Then, if I run across a cart that doesn't have a clip, for pretty cheap, I may just glue it in and see how it works.
Looks like you're not actually using the clip the way you're supposed to. Remove the stylus, hook the rear of the cartridge under the tab first, then swing up the front and snap it in. This will take some pressure. It's a very secure system that weighs next to nothing, and probably cost next to nothing to make. Which was probably the main benefit, back in the day when Shure was putting these out by the millions.
You're right that the clip can be "slippery" under the screwheads, but when used as designed it has a very secure grip on the cartridge.
Bang4buck, are you referring to me or to Donal? I think I have it installed like you have described....the metal clip does seem to grip the cartridge all right, but it's the mounting of the cartridge onto the headshell with that clip that is the issue. Or, maybe it's me not using the right screws or needing some washers etc. etc.
To you, I guess, although Donal may have the same issue. Can't tell for sure from his photos but yours in post #20 clearly shows the cartridge is sitting at the wrong angle. That's what happens when you don't hook it under that tab. The rear of the cartridge body, where the contact pins stick out, must end up parallel (nearly flush, actually) with the "holding arms" on either side.
I know, I know, these look like the bottom of the cartridge should be parallel to the record, but it shouldn't. I'll upload a photo in a moment.

Properly mounted M75, which is the same body as your M91
https://goo.gl/photos/umh7PHsV1s8FKBRK9
Oh, I see, those screw ears are open towards the front, not the side. That can be helpful or a PITA, depends on the situation. Try mounting the clip by itself and then clipping in the cart.
And you're very welcome. You have my sympathy for having to live with those finicky Dual headshell sleds.
Donal, have a look at how yours is mounted and make sure that you follow bangforbuck's instruction.
The part of the clip where the screw goes thru is not a hole, it's more like a prong, and when tightening, the screw tends to "walk" out of the prong. I'm prolly gonna be re- doing this one at some point and use washers. https://www.vinylengine.com/library/shure/m93.shtmlI'm not clear on why you guys are having difficulties tightening the screws.
I'm not clear on why you guys are having difficulties tightening the screws. Are these metal brackets different than the metal tangs on a Pickering or Stanton body, somehow?