I don't see why getting witnesses to sign either a will or a codicil should really be a problem. The witnesses don't have to see the content of your will. Indeed, they don't even need to know that the document that you're signing is a will. They simply have to confirm that they saw you sign it.
My current will was witnessed by a couple of mates sitting with me in a pub garden. The previous version was witnessed by a couple of strangers, who I happened to be working with that day, in a Tesco car park! Any two adults can do the job for you as long as they're not beneficiaries of your will.
(It's best if the witnesses print their names and addresses below their signatures, so that they can be traced if there are any queries about the authenticity of your signature. However your will remains valid even if they can't be traced because, say, they die before you do).