You can remove the short one and replace it with something like a Bibia. They're cheap and pretty big, although ultimately any rubber sheet would do.
Is it much bigger than this one? Can't tell from the pics. (Dunno if you can tell from this pic)
Went for a lunchtime ride out to the chalk pits. They're full of chalk. I asked if they had any dry wipe markers but they looked at me weird.
You should level your saddle a bit....
It is (175mm x 130mm). You could bol or rivet it at the bottom of yours.
@Belagerent started
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You can remove the short one and replace it with something like a Bibia. They're cheap and pretty big, although ultimately any rubber sheet would do.