Thanks all! Forgive my slow reply as I dropped off the grid to use the van....without the fan, and need to get back to this mission. My setup will likely be like JackGrimShaw's and set far back like connectvanplans, but offset. I have unfortunately received no confirmation that the adapter for the newer Transit will work in offset mode. It can surely be done offset and I've seen it. At current I will just go with butyl tape and a wooden frame inside unless a magic answer appears. Many folks who have put in center came to the realization of impact after. I haven't started drilling or cutting yet though!
@JackGrimshaw is that the Skybox 12?
I will share back when I've made the cuts and installed.