The problem with the transom
It's not difficult to throw something over the transom. That's not the problem.
The problem is it's a waste.
The internet has made it so easy to wrap your idea/proposal around a brick and throw it that we forget sometimes that just because you can, it doesn't mean you should.
What sort of proposal should you write to be sure that someone who gets it over the transom will read it? You shouldn't.
Instead, spend the time earning the right to make the proposal. Spend the time building a presence that gets you an invitation, or, at the very least, earns you the credibility to walk in the front door. If you want to pitch a great business development idea to a big company you don't already work with, allocate three to six months of focused, patient effort to earn the right to make the presentation in the first place.
AND! If you are an organization that has a transom, how can you make it easier for great ideas to arrive? How about publishing your org chart so people can find the right person to contact? How about publishing a list of others you've worked with and what you've done with them so others can see what you're interested in? How about appointing someone (an agency? a bureau? an individual?) to act as a filter for you so you're not randomly fishing through the slush pile now and then?
A few years ago, Joi Ito used to use his tribe to review business plans. You posted your plan, they (the motivated volunteers) would mark it up and Joi would look at the best ones. It's not clear to me that opening the transom wider brings you more crackpots. In fact, it may very well shed light on great opportunities you've been missing.