If you're really set on getting some sort of prefab for temporary (and possible later) use, that's on the cheaper side, I'd go with something like a rabbit hutch that can be placed at ground level so the adults can look in and see the chicks to start acclimating themselves to one another. I'm talking something like this with the legs cut down: https://www.wayfair.com/pet/pdp/tucker-murphy-pet-ennis-rabbit-hutch-w003192443.html?piid=193593142
My outdoor brooder, for reference. I converted this from my old prefab coop:
My outdoor brooder, for reference. I converted this from my old prefab coop:
