Slipping seatpost clamp

I've recently been doing some light touring around Europe on my Moser and an Apidura saddlebag. It's been working consistently so far, but since getting to London it seems to have lost the plot.

With the saddle bag loaded I found that the saddle was being pulled back over the course of a days riding. This was annoying but bearable, and I could simply reset it at the end of each day. However, the saddle is now moving backwards concerningly easily, even when not loaded. Having a closer look it looks like the top clamp is no longer fitting flush around the rails and is preventing me from tightening it properly.

Now this all wouldn't be a problem if I had a normal seatpost. But I'm not. And the quill seatpost that I have is not going to be replaced easily.

Long story short, does anyone have any tips for stopping the saddle from slipping? I would rather not resort to replacing the entire post with a conventional post, along with the frame modifications that would be required to make it work.

Cheers

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could you modify the top of the clamp somehow? It's hard to understand what the problem is, maybe some better pics?

I would probably have a go at it with a file.

On the other hand, frame modification should be pretty easy, all it would require is drilling a hole and cutting and slit for compression, then using a seatpost clamp. You could probably even touch up the paint with nail polish and be done!

Are the parts that hold the seat rails bent? More likely the bottom would bent than the top but you never know.

If so, find a similar/identical part from an ordinary post and swap.

Consider shimming the rails/clamp with either beer can (superglued in place) or a small piece of inner tube.

Totally missed these replies... I ended out finding someone who didn't mind me making a mess of his workshop and made my own quill seatpost. A 1000km or so later it's still working, which was a nice surprise.