Treat it just like a 30-second ad. Sell your channel to the viewer. Once they subscribe, they don't see it anymore so it is just for new viewers. Have a SINGLE call-to-action at the end and that being for them to subscribe. Nothing more. Do NOT monetize it. Don't let another ad spoil your ad. This is just to sell your channel and not some advertiser's stuff. While viewers will usually find your channel with a topic search and even subscribe to it because of its topic, know that it is YOU that keeps them as a subscriber. So first talk about your topic and why they should listen to you about it and THEN talk about yourself as you relate to your topic. Use this as a chance to humanize yourself to them. And before asking for them to subscribe, tell them your release schedule so they know what they're getting themselves into. Lastly, don't be afraid to have it run up to a minute. Just keep it concise and brief as possible. It doesn't have to be exactly 30 or a minute but somewhere between.