I'm steaming my seitan (after I wrap it in foil), but it's hard to get it in perfectly round sausage shape. On the other hand, the seitan sausages I buy in stores are always perfectly round like a cylinder.. How do Seitan manufacturers get their seitan in this cylindrical perfect shape?
The easiest way to make perfect sausage-shaped seitan would be to buy vegan sausage casing(assuming you are vegetarian or vegan since you are making your own seitan).
Something similar to this.
Seitan in general isn't really smooth. And generally if it ever becomes smooth its probably over-mixed and becomes really tough. The casing provides a smooth surface and helps keep it in the round shape while you boil or steam it.
What you can do is foil-wrap them so there is some room to expand but a strong constraint once a certain expansion is reached. A couple of layers of tinfoil that are wrapped loosely, but twisted/clamped together firmly at the ends, will work well for baked seitan. Expect it to sometimes burst the foil, won't spoil it but don't make a setup where an accident can result from that happening.