I have just made a case for a ukulele using stiff cardboard and strips of soaked paper, it was a success, but it took far longer than I anticipated.
Before you start, consider the following;

After you have made the main components,

1. Marrying the lid to the case and fitting the hinges and catches.

2. Making and fitting a handle.

3. Forming the internal box for plecks, strings, ect.

4. Fitting out and lining the inside to take the instrument.

5. External covering.

And when you've done all that;

Be prepared to make to make further adjustments, because the whole thing tends tends to warp quite badly as it dries out.

All in all, including drying time between jobs, it took me 3 days to make. Next time i will just buy a case off the shelf :)
Fun to make though!!
John.