I have never made one but, I know others have. I have been thinking of making an urn for myself.
I would start with going to the sorce, a funreal home, and looking at what they have to offer. I know that the interior volume is important. You need enough cubic inches for the ashes. Volume does change according to the physical size of the person and it is not nessaraly just all ashes. There can be some bone to account for.
It can be any shape (round or square) and must be designed so once closed can not be re-opened.
There are some laws governing what can and can not be done according to where you live.There are so meny different laws and rules and alot of hearsay about what you can do and not do.
I have "heard" you can't scatter ashes above ground, you must bury the urn or put it into a colemberrium, you can not keep the urn in a home etc. I think most of that hearsay is false but, I haven't checked any of it out with my local laws.
I'm sure that your local mortition can and be willing to help you out. But rember, he is in business and may feel threatened by some of your questions.
Rog