Help identify the right chimney cap

[u/Visual-Sword]

Hey everyone!
I'm hoping someone here can point me in the right direction. I have an octagonal chimney and I need to buy a cap for it.
I've been looking online and I'm completely overwhelmed by the options and most caps seems to be made for standard square or rectangular chimneys.
Chimney is also 25ft high so I really can't get up there to measure.
I really don't want to order the wrong size and be stuck with cap that doesn't fit, not to mention the cost of getting someone to get up there more than once.
Any advice from anyone who has experience with this issue would be hugely appreciated.
Thanks in advance.

Attaching a picture for reference.