You definitely should not make up a barcode. The proper way to do it is to register with the Uniform Code Council, but that's expensive.

The alternative route is to go to one of the many companies that will sell you one of their barcodes.

https://www.indierec.com/s-barcodes-register.html

their barcodes are primarily sold for indie bands and so forth, but would work fine for DVDs.

More and more retailers are requiring barcodes for inventory tracking, so it is a good idea to have a proper barcode.