For much less than that, the OP can use a USB flash drive.The ease of using GoSecure is worth $20 to me.
Yes, he can do that. But he asked about encryption options, not a way to hide the file. If he decides to hide the file instead, I would recommend saving it on at least two drives since USB flash drives are not fool-proof data storage devices.For much less than that, the OP can use a USB flash drive.
Copy the tax return files to the USB drive, then delete from the builtin disk. Eject the USB drive, unplug, and hide somewhere his wife won't look.
For safety, make two copies on two separate USB drives. In case the dog finds one and thinks it's a chew-toy.