We're currently planning on our JH concert being outdoors under the roof of an old cafeteria that they left standing after they removed the sides as a pavilion. We'll premark squares for families to setup folding chairs or blankets that day. Our backup plan is to have the kids perform and livestream it with no audience.
If you are going to have an indoor event I would limit capacity to 25% and require masks. Temp checks would be required in my district, but I put that more in the 'security theater' category.
To be perfectly blunt, numbers are rising across the country and Texas, and you should presume with a group of 100-200 people in a space you will probably have at least one positive individual at this point. The risk factor to me is not worth it in the slightest! By the time we reach our concert date we'll be in the midst of the biggest surge from Thanksgiving - I won't be surprised to see a lockdown by then, or at least limitations on gatherings at the minimum.
As far as preparing for concert and area? Good luck... If we had our Area contest on 12/5 as originally scheduled I could have put together some easy pieces in time for our concert. Ours is now set for 12/12 though, and we're just going to not have a winter concert at this point. I'm bummed in a lot of ways - I am more than ready for sit down band! The thing is we're probably safer rehearsing a marching band outside anyway.
Best wishes to you and your students. Stay healthy!