Hey, can you post a pic of where you can get those covers ? Sorry never heard of one, do not know what they look like. I agree the bulldog cover sucks BAD. It was cold and I broke it by forcing the cover off. My find motor skills were not there since it was cold, and just wanted to hurry things up ,and guess what yep broke one of the tabs on the cover.
IF I can find mine ( dont hold out but I will look around ) I will send it too you, junk. I just went to thye auto parts store and purchased 6 rubber covers for a radiator over flow/vacuum port cover. 6 for $1.34 including tax.
If you have paintball shops they all carry the magnetic covers, try googling magnetic paintball fill cover and hit images, should show a bunch.
That cover is the one huge boo boo on the bulldog design.