You are welcome.
It seems that there are some programs and webpages with helps you to perform the checksum and select a 24th word which would make your seed a valid one, however that may imply you would need to expose your seed to the internet, making it unsafe.

My recommendation would be to use an open source code to make the checksum and run the process in a computer without internet connection, once it is done and you have your 24th word, proceed to delete the code. I am not sure how safe this will be though, It would be constructive to listen to other's opinion on this matter.