I picked up your question on the above. and thought I would get in touch.
I am the person referred to in one of the replies. I made one some time ago and it is very successful for both hot and cold drinks. When I waqs at the Loch Achray meet I just had a leather thong on top with a dog clip to attach it to my belt. So0meone suggested it would look better with a cup handle. I have since carved a handle out of mahogany and I think it looks much better[fitted just above half way up the side.
I like your first attempt. At least you got a straight line across the nut,
Mine was rether wavey to start with and needed a bit of filing down to straighten it up.
I would however suggest that next time You cut the nut much nearer the top like taking the top off of a boiled egg, just above where it starts to narrow in.
Do not worry about a flat bottom as I find that with the point at the bottom it sits much better on the ground on grass than an ordinary cup or mug. If you wish you could make a plaited woggle or carved wooden ring to sit it in if you wish to use it on a flat surface and just carry this separately for when you need it.
As far as making the cup goes, Just rasp or file all the copra fibre off of the outside but dont go too deep coarse sandpaper is probably best but takes longer.
When you find that you are down to a black or dark brown surface all over with just a few veins of fibre leaving a pattern, go on to a finer and finer grade of paper till you achieve a fine polish [it really comes up fine and shiny]
I do not consider that the inside needs any preparation as it was designed to hold liquid in the first place. I did houever give the inside a coat of one of the new plastic varnishes recently as i had managed to crack the shell. When removing the nut material from the inside I would suggest carving or cutting a small wedge of the nut out before trying to pry the rest loose so that it has space to move into and does not force the hard outer out the way and thereby crack it..
AS for the handle i just carved and filed it to meet the curve of the nut and glued it on with super glue . If you have any further questions please come back to me.
I do have photographs of the cup but as yet I do not know how to send them.
Best of luck with your project hope this may be of some help.
Cheers for now . Stuart S