I feel that some absolute time limits for napping is a no no for me. I grab my keys to my other hand and I extend that hand over the bed. It takes some time to get used to the idea of having the keys on your hand while trying to nap, but soon enough you'll forget you're holding the keys on your hand. Eventually you'll drop the keys at the exact moment you doze off and that's a clear sign from yourself that you've hit the time limit of a nap. There's no need to set your alarm clock for a 20-30min nap.

The alarm will inevitably go off during the wrong moment.

If you try to keep something in your hands (which will make a sound once fallen) you'll get the alarm at the exact moment you strive for.