I note Journeyman's use of d.p.c. as I have some remaining from replacing my workshop windows, and it would probably work very well on the cross-slide of my Harrison L5, where space at the back is a bit too limited for a rigid cover.
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My Myford VMC mill came incomplete with the bellows between the back of the cross-slide and the top of the column.
After I had made the garden frog colony happy by building a deluxe pond especially for them, I tried a sheet of left-over butyl pond-liner, not sure how it would withstand oil and folding.
It is not in bellows form but works well, seems oil-resistant, though does bulk up a bit at extreme set-ups (sets-up?) and travels, and I have had to remove it once or twice for such tasks. Those are rare though.
To hold the top edge I used a piece of thin aluminium angle from B&Q, drilling the resulting ledge with several holes of various sizes to hold the centre-finder, drill-chuck key, screw-driver and such-like.