I haven't tried to go down this route, but I'm also not at the level that I think makes sense.
I have worked with some motorsports drivers with developing their sponsorship proposal, so I have a bit of experience in that regard. It all comes down to two things: eyeballs, and ROI. You need to show the potential sponsor how many eyeballs they can expect to be in front of, hopefully eyeballs that are potential customers of theirs. From there, you need to show them how a small investment in you, either via cash or via product, can return an even larger return on that investment. In general, a minimum of 3x return should be displayed, with 5x to 10x being ideal. You should also showcase some of their other advertising options, to show how the ROI gained from working with you either hits a larger audience, is available at a cheaper rate, or ideally, both.
All of this will be extremely customized to the potential sponsor you're pursuing, so even if someone had a boilerplate email template, the details would still require a substantial amount of work on your end.