Well if someone is not capable of filling out 2 textboxes in a UI (one with the words and the other with the passphrase used) then they also won't be able to use any other method such as your shift cipher which requires the same 2 inputs (mnemonic and a date)!
And risk using these online encryption/decryption services who may save the results and steal funds as well? With my method you can do it by hand, you don't need any script.