I can't physically hold the required position comfortably.
The exact position is not actually critical. It's important you find a position in which you can be both alert and comfortable, and that may mean modifying things somewhat. It's not expected that you be absolutely still during the practice of meditation. If you need to shift position, that's no problem, but it's a good practice in mindfulness to first notice the urge to move before actually shifting position, hesitate for just a moment, and then move, if necessary. This makes it a mindful movement and not an unconscious one.
For a "sitting" meditation: If are sitting on the floor, it can help to elevate your hips with cushions or a meditation bench so that your bottom is higher than your knees. If you are in a chair, you can lean back and rest or not. Sitting upright however, will help you stay alert and aware. If you nod off while sitting - you could fall and hurt yourself. If a sitting position does to work for you - try a recliner or using the wall for support. You can also lay flat. The main reason this position is least taught - you may fall asleep (which honestly, isn't a negative). You can even meditation while standing. Remember, practicing the mindful meditation is what is important.