Postal Procedures
It's been many years since I had a postal vote (in the days when you needed a good reason to have one) but surely the procedure should be to have two envelopes. Mark your ballot paper and put inside envelope #1 which has a serial number on it (which may or may not match the one on the ballot itself). Place envelope #1 inside envelope #2, seal it and fill in your identifying details on the back. Put in the post.
At the receiving end, the up-front work consists of checking the details on each envelope #2 and if OK, remove envelope #1, make sure its serial number is correct and the envelope is sealed and place it unopened in a secure ballot box. On the night, open the box in the counting hall and remove the ballots from the envelopes, having first noticed that they are still sealed and include them in the count.
Perfectly secure, no chance of a leak provided the procedures are followed. No easy way of identifying individual votes either, apart from the mechanism built in with the ballot serial numbers that is common to all of them.