My favourite are the JHL (Jumpers Horse Line) and Mark Todd neck covers (they are made by the same people)
I do also have some Masta neck covers, which are very nicely made, but quite long in the neck so my boys tend to have them over the top of their ears, which must annoy them!
Has your current rug already got attachments for a neck cover? i.e. what sort of attachments must the neck cover have?
I've got a hardly used Olympian neck cover which has a faulty clip on it (could be fixed by anyone with needle & thread that can be bothered to repair it!). I have absolutely no use for this neck cover & would be happy to part with it for not much more than the cost of the postage if it would help you...
Whichever neck cover matches the rug - so my Amigo rug has the matching Amigo neck cover, my Belstane has the matching neck, then the HW has an integral neck.
I like to use the matching style to make sure it fits properly. I wouldn't be able to swap neck covers on my rugs as they don't fit the same so would leak etc.