Basically there are usually 2 common types of saddle clamp,
First is the 'snails back' type, which is notorious for slipping (tilting) as it appears to be one shiny surface sliding off the other shiny surface.
Second type is the 'cupped hand type' which normally have serations on both mating surfaces.
If the former, then I have had some temporary success, by taking the clamp apart, then with a hack saw, make a series of scores across the surface, in all directions, on the 'hump' that is the top part of the post then with some very rough sandpaper, rub the inside of the upper section, to roughen it up. This should provide a better grip, and when you use the allen key, use something over the end to extend it, and use one hand to push the allen key up into the bolt.
Alternatively any seat post with the cupped hand and serrations should work fine ( 27.2mm dia presumably). I'm sure if you put a wanted add, you could pick one up fr the price of postage. Just make sure its long enough.