Why not just focus on doing one maybe two of the active exercises for the time. Set a timer for the full time. About half time when it feels right to you switch exercises if you're doing two. You can do just one of the exercises for the full half hour. If you switch between two it doesn't matter if you spend more or less that 10 minutes on one.
I know I don't have a problem with this because I wear a watch that vibrates with 4 individual timers. You could use a motivaider which can count off set blocks of time.
Or a mobile phone with a vibrate feature for memo reminders might work.
Though I think it's easiest to stick with one or two of the exercises for the 30 minute block and trust my intuition to change exercises if I'm not using the tape or CD
Alex