A Hawke 50mm AO is different size to a Hawke 50mm standard non adjustable scope, and more than likely different to most other 50mm scopes. I would bet that the Hawke 50mm AO sunshade is bigger than actual 50mm, it is just that it is made for that model.
You really need that specific sunshade. If you click on the sunshade to buy it on the Deben site they will notify you when it is available. They will be the first to know as they are the importer. I did this with an out of stock BKL mount recently.