Starting about ten years ago, I had an Integral Designs SilShelter. First I sewed on about a six-inch skirt of bug net, and that worked good. I needed something around the front door opening, so I simply layed a large square of bug net over the front. It slid off, so I sewed some velcro onto one corner and that was a little better.
Starting a few years ago, I had a Gossamer Gear SpinnShelter. First I sewed on about a 4-inch skirt of bug net, and that worked good. Before I took it to Alaska, I sewed some bug net inside the front vestibule. That is in two triangular panels of bug net that overlap slightly and hold together and onto the main tent fabric with velcro. That works, and I removed it after leaving Alaska.
Now I am dealing with two new shelters. One is the HMG Echo 1 Prototype that was mentioned here a week or so ago. It is slightly more of a tarp. The other thing is a strictly MYOG tarp. So, neither of these has a vestibule, per se. These two new ones are slightly more open in the front, which is why I was interested in getting some kind of rain flap for bad weather. Each of these two might also need some rear bug net or rain flap also. Being weight conscious, I would not want to create bug nets that weigh more than an ounce or so, and similarly for a rain flap, not over an ounce.