Might think of making it a screw on lid. Why don't you want to solder, weld the lid on after the ashes are in the urn? It will not "hurt" the ashes and as your Father was into working metal, maybe welding the urn shut would be just the thing to do. Or your could crimp the lid closed. Also there are a number of sealing materials (mostly silicon) that could be used. Also riveting comes to mind, using a sealer to really "seal the lid" as/before it is riveted on. If you are into crimping, this too could be a way to seal the Urn. Use of a tar, either "real" tar or water thinned or thinner thinned tar or wax might be considered as a sealer. It just depends on what you lean towards and what type of closure you are interested in making. Of course you could always seal and bolt the lid on. Stainless steel would give the longest "life" for the bolts and nuts, gold would be the longest lived.

Hope this gives you some things to think about. I am sorry for your loss, but your spending time asking questions about this and doing "your correct thing for your Dad" is absolutely wonderful and a great way to honor your Father!!

My best thoughts and wishes are with you.

John Dach

The first thing,sorry for your loss.
Before anything I would want to find out through the funeral director what the total volume of the urn needs to be so you can plot depth,width and height.
The next thing would be to decide if you want either a traditional round urn which would require metal spinning equipment and skills or a square or rectangular box.
Either way,rather than fabricate a separate cap (because you said you don`t want to solder later)I would fill the container through the bottom and then have the bottom slide into place and seal it using something like a folded seam.If you have all the gear then this should be a fairly easy task.
We planned on a cloth bag that served as a liner for my father`s urn and closed the bag with a simple draw string.My mother made the bag.
We also opened the making of the urn to as many of the family members and close friends as wanted to participate and made sure everyone had some way to contribute to the project,even if it was just to hold the urn while others worked on it or to hand polish and wax the final results.That way everyone who wanted to had a way to honor my father`s passing and more importantly became a participant in rather than an observer of the ritual.

In theory there is no difference between theory and practice.
In practice there is.

I am also sorry for your loss. I make foldformed boxes that are riveted and have a bottom plate (with feet) that is dropped in and then crimped (no solder). You could build the box upside down, top riveted or crimped on, center section riveted at seam, ashes put in and then the bottom crimped in place. All patina work can be done to each section before being put together. This would work if the ashes were in a liner of some sort. If it needs to be air tight I would seal the seams with silicon after the patina is done.
Here is a photo of a very simple design showing what I am talking about.
Hope this helps. Blue dome top box: Simple foldformed,domed top copper box

I actually have done 2 of these- but neither one was inspected by government or anything.

The first one was a copper box, 1/8" sheet, tig welded seams, and it had a flat cover panel that screwed on with countersunk brass flat head socket screws. I think I might have used some sticky back foam door sealant strip between the two pieces of copper, so it sealed tight when it was screwed shut.

That one has two people in it, and has been aboveground, outside, for over ten years now.

The other one was made from aluminum pipe, with a tig welded bottom, a tig welded flange, and then, again, a top that bolted on.
I had a cast aluminum light bulb, left over from another project.
Again, I dont know about any rules- but Sie, the father of a friend of mine, is in there.
urn

I have no advice but just want to say that this is a wonderful thing for you to do. Good luck.

marilyn