Whilst giving my Dolly some cavity wax protection for the winter, I noticed the near side top ball joint has a split in the rubber which will be an MOT fail.

I see on Ebay you can buy replacement split dust covers and glue them closed, thus avoiding the need to remove any nuts... Are these any good?
I think I would prefer to replace the dust cover as a whole unit, therefore can anyone tell me where I can source some covers of the correct size/type? Or failing that, tell me what the top & bottom hole diameter should be?
Thanks in advance,
Dave