The flies cruise a bit above ground level looking for carrots so planting in fairly narrow beds with an eighteen inch poly 'wall' around them means they lift to get over the wall and miss the carrots before they come down the other side. They say a carrot fly can smell a pulled carrot from up to a mile away, so do thinning in late evening, preferably in the drizzle.
I have been building a single size raised bed as a standard in my garden. 30" wide and 95.5" long. And my goal is to have one crop or type of crop per bed per year.
1) I think I could easily make a frame to fit over a standard raised bed and have floating row cover or light weight shade cloth on that frame.
2) I could make the poly walls but then I risk trapping in a fly and giving it a feast of carrots.
I like the idea of thinning late in the day. I think I also heard something along the lines of this and NOT planting radish to define the row as pulling the radish will also disturb the carrots and ring the dinner bell for the carrot fly.