I’ve often run in to trouble sewing a tube to a disc. Like the bottom of a stuff sack. In your case this would be the floor to your inner. By the time I’m back around near where I started, one side has ended up longer or shorter than the other. This could be because I’m just not quite good enough at it, or maybe it just isn’t the right way to do it. So I favor sewing around the perimeter first, and then closing up the length of the tube. By leaving a little extra on the circumference of the tube, I can adjust to where ever I end up once the end piece is sewn on.
If I was attempting your inner, I’d cut out the triangular wall panels and attach them separately to each side of the floor first. Then sew up each vertical seam to complete the cone. If you’re making a bathtub floor, don’t make the vertical seams at the corners before attaching the walls. Give your self a giant star shape made of the floor and the walls, and then close up each edge.
If your door is fully contained with in one wall panel, do that first. If you want to put zips on the seams, you’re probably better off sorting that out yourself.
All this applies regardless of whether you’re doing a full inner, or half. It still amounts to a cone of sorts on a base.
Regarding re-enforcing the vertical seams in the outer fly, grosgrain ribbon may be easier to sew on than cord. With a 40d, I don’t think you’ll need it. But you might make a point of leaving the seam allowances extra wide and folding them over more times than normal. This will have a similar effect to sewing on ribbon or cord. You might also want to consider that there’s less fabric at the tip of the pyramid to carry the load. Adding a second layer of your wall fabric to a small area at the top addresses this. Take a close look at some of the photos of MLD’s mids – they use a heavier fabric re-enforcement at the tip.
Good luck!