First, Have you already picked up your hull? There are now two excellent sources. But since you are asking for Kevin or me, you probabably Know that.
Kevin is using this:
So, a 3.5 WTC of a home made design should be about right.
When mine comes up in que, I will likely use a 3.5 also. I am still debating an RCAB system, or a custom pump design like Subron 5 member Gail has built from scratch.