Hi,
If I had to do this I would have done it as following:
1. Buy (or borrow) a sword bag you like.
2. Take all the measurements.
3. Get a suitable piece of cloth.
4. Cut the piece(s) and sew them together. It would come handy if you know someone who owns a sewing machine.
5. Sell the original sword bag.
Myself being lazy I would have probably stopped at p.1
BR/Andrei