For what it's worth, I built a TEM cell based off this design last year and had great results with it, so thank you! We did a few rounds of half-days at the EMC lab, and each time I could correlate the results pretty well to what I was getting in the TEM cell. It was much more repeatable than the usual near-field probes, so I could try out a bunch of different PCB/enclosure modifications and re-test with good confidence we'd see the same relative change in results at the next lab session.
I did make a few modifications to the design when I built mine: I changed the ends similar to the TekBox design so they would clamp the septum rather than rely on the solder joint to support it, and I didn't do any cuts in the septum. I made the septum out of single sided copper-clad, cut to shape on a bandsaw, and then sanded away the extra copper at the ends. I couldn't get a large enough single sheet so I just used copper tape (with conductive adhesive) to stick them together.
There was a mistake in mine where I tapered the septum copper down to a point but the shield doesn't taper right down, so there is some impedance mismatch at the ends (you can see the inductive bumps in the TDR plot below), but it worked so well for emissions testing I never bothered re-making it.