I don't have a portfolio, but I'd be happy to work on your prototype. As long as the instructions are clear and expectations are real, I see no problem in me getting the work done.
The reason they want a portfolio is because they'll be investing time and resources into you and while you may be confident in your ability to deliver, they're probably not.
If you end up failing to deliver, it would be wasted time and resources on their part. The point of a portfolio is to show that you have the skills to do what they're looking for.
Don't let the other guys drag you down - they sound bitter. But from their perspective, you're not giving them anything to work with. Fill the gap pronto! :)