You're overthinking it. If you have intake fans on the front and exhaust fans on the back, most of the air will flow that way, even accounting for distortions from GPU and CPU. If you have intakes on the side and exhaust on the back - it will flow
that way. Check out airflow simulations on youtube, or, if you have spare time and really curious, run some yourself. In other words, there's no need to seal anything or create special side intakes unless it's for some other reason the exact thing you want. Practicality-wise you're not winning much by doing all that.
In any case (lol), Phanteks (
1,
2) has some full metal cases and Silverstone has many more. Like
this one. If you really want something specific, then yeah, you'll have to google through hundreds of old designs or ask someone to make a custom one for your specific needs. Which would be really cool, actually — if you have money to burn, having custom shit built for you is amazing + you get to support a small business in your area.