Welcome Mountain Meadow. When I built my shed in the back forty, I designed it with the specific purpose of being an all-year boolit casting center. That said, I installed an unused bathroom fan over the casting bench (like someone else said) and run the exhaust through a homebuilt HEPA filter. There are two ways you can pick up lead; vapors from lead that is overheated, or dust that is on surfaces you touch. Part of the recommendation for not casting indoors is that no matter how careful you are, some lead will always escape and be deposited in the vicinity. I continue to find lead splatters around my casting bench so I confine my boolit making the that location. Smoke is another matter. If you are rendering scrap lead, some if it can be pretty well contaminated with stuff that smolders rather than burns. Stuff like that I do outside.
Wayne