Does anyone have experience writing a Certificate/Statement of Non-Availability?

Call housing if in doubt.

I have wrote one once and it was denied. If you contact branch or a good S1 the will have an example memo. Usually you will have to submit supporting documents. I wish I still had mines I would email it to you. I know it's a pain in the butt, hopefully you will get yours approved.

Army Regulation "How to Prepare Army Correspondence" Can't remember, however, I googled it once and was able to obtain it.