Cover letters like all things can be simple and not so simple. I always try to create a cover letter with a little blank in it that I can add personal information if there is any. If you can connect with the client right from the cover letter, that's a great start.
My cover letter usually highlight my home studio equipment, my connection capabilities (ie: Source Connect, ipDTL, Zoom, etc). I my location with a time zone (many overseas clients out there). I also inform the client whether I am willing to travel to a studio should the need or opportunity arise. As lockdowns ease, more and more recordings are moving back to downtown. I let them know my recording schedule and any potential disruptions. No one likes it when you book a job and then are unavailable to record because of a previous engagement. If you are upfront, the client usually work around it.
If the rates are clearly stated in the brief, I will acknowledge them and further mention my rates on future jobs are negotiable. It is not advisable to put your actual rates on a cover letter in case you get locked into something that could have paid much more. Always be open to negotiate.
Near the end, I will inform the client that if another custom demo is required, they are welcome to get in touch and I will provide one usually same day at no additional charge. this is important for me, as some VO do charge for requested demos and I do not.
Finally I thank them for considering me and my demo, and provide a link to my website that has further demos and information.
I do a lot of copy and pasting, and if there are names of clients I can address, I add them in at the top. Again, like everything else in this business, not all cover letters have to be dull and boring or extremely business. If the demo is light and funny, consider a light and funny cover letter, still providing the necessary details. If it's for a funeral home? Deaden things a little.... did I go there? sure I did.
If the cover letter is 500 words or less like Backstage? Hello, insert equipment. location, and a thanks. If you can point someone to a personal website, all the better for lack of words.
Have fun storming the castle