I completely agree with TsPoet. It is not at all unethical to write your own letter so long as the boss really does read and sign it. View it as a request from the boss out of laziness. It probably is. I've never had to do it. It's uncomfortable but a common and reasonable request.

And do NOT sell yourself short. Don't be modest. You can email it to the boss so he/she can edit it. Then it really is just a time saver for the boss. Thats all.