Modify your Polaroid Spectra to use film holder?

I ran into this Instructable: Modify Your Polaroid Spectra Camera to Use Non-Polaroid Film by Nano_Burger
https://www.instructables.com/Modify-Your-Polaroid-Spectra-Camera-to-use-Non-Pol/

I happen to have a Polaroid Spectra, though I don't have any film or old film packs for it, so this sounds like a fun idea...

I am wondering if any of you have tried this mod?

The issue I see with this mod is determining where the actual focal plane should be in order to get precise focusing on the sheet film in the holder.

I've thought of doing something similar with an SX-70, I find the idea of a folding SLR really appealing. But it's not something I've tried (yet).

I don't have a Spectra, but considered doing that with an old One Step, I even got a small film holder. Then Impossible started making Polaroid film again, so I didn't. Still, I'm intrigued by the idea. If you do it, be sure to keep us posted.

I'd like to make my own film holder for the Spectra, but I don't have an old film pack to measure- maybe someone can provide me with the film packs measurements?