I live the the lower part of Upstate South Carolina and have been in the coastal areas MANY times kilted, as well as in VERY humid 95-105 (F) ACTUAL temp.
My favorite kilts are 16 oz worsted box pleats.
My Caledonia and X Marks are F&K 13 oz merino worsted (feels lighter than 13).
With the breezes, I wouldn't go lower than a 13. 11 is POSSIBLE, but I prefer heavier than 13.
I WEAR my 16s most of the summer and switch to 13 based on tartan/colors NOT on heat.

This past summer, I was on I-85 in downtown Atlanta (actually I-75/85) coming home from the main office (which is in downtown). I left around 2/2:30. The air was made hotter by the fact I was near the wall of a busy 6 lanes per side interstate (engine heat and asphalt radiant heat amplifying the "normal" air temp). My guesstimate was that the air around me (in a convertible) was 120-130 (F). At 5:30 in South Carolina the temp was still 105. (This is as I recall, I think I told the story here before-> THAT version is fact on the 5:30 temp.)
I was in my 16 oz wool Wallace kilt and fairly comfortable.

Worsted wool breaths amazingly well, as long as there aren't too many layers. The "actual temp difference" between a 13-16 isn't that much (on a box pleat), while the 16 hangs better and stays down better in a breeze.

8 oz is just WAY too light. It will blow up easy and won't hold a pleat all that well.
I'd say an 11 AT MINIMAL and recommend a 13 oz (if you can find a good deal).
If you use an 11, overlap the back/insides of the box pleats (takes more material, but adds depth and weight to the boxes) or make a 5-6 yard knife pleat.

For a first attempt kilt, I would look for a decently heavy material and make one. The main thing is that it be decently heavy (to hold itself down in a breeze) and be a twill weave (for strength-> affects holding shape and keeping everything "right" and down). My PERSONAL preference is 100% wools, though blends work OK, as do cotton/poly, acrylic, P/V, and various blends. For the first, I'd not worry about a worsted wool or an expensive fabric. Many decent ones can be had for $10-15/yard (or in that ballpark).
If you like it, the next will be significantly better as your skills improve. The first couple will gradually become "beater" kilts. My first 2 are not worn much outside the house, while the third is part of my "normal wear" and my fourth is one of my absolute favorites and get worn a BUNCH.