What I would add is that I'm currently using an inverted copper cylinder, and consequently that does not have a domed top. It has the vapour takeoff via a pipe that goes in through the side then up to the rim of the base. This works absolutely fine, so there is no reason why it wouldn't with a square vessel.
If you look at the GL push-pull design, he has (stainless?) wire wool in the vapour takeoff to collect any drops of vapour and drip them back into the vessel. For us this isn't really necessary.
My new processor, like KH, will be round, just because that seems right. But for me, that's all about the domed drain-off more than anything else, great for getting the glyc out.
Everything else will be square though, I'm so tired of drums wasting space. I have lots of vertical space so the steel tanks I've acquired are ideal, as they're nearly 600l but with a footprint of just 24 x 30 inches (tall and not wide!)