This is kind of dumb but for any common interval length there is probably a YouTube video that's long enough for your workout. For example, 30 seconds and 1 minute: https://www.youtube.com/watch?v=t_Bb4PfLvJE, https://www.youtube.com/watch?v=Y-AYC3_DbpY. Save the ones you need to a playlist and they should be pretty quick to access.