I'm sure that many of us have thrown hundreds of those out! Unless someone happens to be sitting on some, I would think your only hope would be eBay etc. However, I don't personally really think that the absence of the cover is a major issue.
As I said, I personally wouldn't bother/worry, but if you're really concerned and can't find one, it probably wouldn't be too difficult to fabricate some sort of cover - as you presumably know, they attach just with a single machine screw which goes into a tapped hole in the casing.
The later fuse covers had large 'knockouts'/cutouts for those wishing to change to plug-in MCBs (which then stuck through the hole).
Indeed, the OP would only need to glue in something to cover the large hole that had been 'knocked (or cut) out'.
I don't doubt that all that is true, but it doesn't really alter anything I've said about how I would personally feel about the absence of the cover.
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