For lupines, it is a bit late. This year, the weeds that grow faster will win. Green manure is not expensive. Only for plowing under I would offer a farmer some money at the time when you see them driving with the appropriate equipment anyway. He does it in half an hour and you only have one set of bones. I would carefully consider if half a day of rotary tilling is really necessary. Crucifers are good, yellow mustard or something from the radish family. Green manure is often a mix; there is not only monoculture.
I would buy at the agricultural supply store, where farmers also get their seed. Alternatively, you can ask a farmer. If it were my property, I would apply the green manure pronto, sow a little denser than necessary so that it covers the area densely, and leave the planting until frost kills it. The next spring, plow it under. If it has reached seed maturity and if the construction site is still fallow, it can simply grow again and cover the soil once more. Of course, you can also sow again. Just not too much hassle: it has to be economical in terms of effect, time, and money. Annual wildflower meadow: Later, in the early time of gardening, many poppies came up behind it that had to be managed. But in the first year, a joy for all residents. Many greetings from Gabriele