It's easy to convert something into a nestbox. Here is one I whipped up yesterday, for the meat hybrids. Since they can't go up the ramp into the night-shed, and their own roost area isn't ideal for laying in, I had to make something that could stay outside. Plant pot plastic is useful for this type of thing because it lasts a long time, unlike some of the more brittle plastics (like ultra cheap dog kennels).
The whole thing consists of 3 large tub-style plant pots sitting in a row, open to the front. The pots are screwed (heads facing inwards, sharps pointing out into the timber) onto the front piece of wood which acts as a stabiliser and foot-hold as well as retainer for the nestbox litter; and there's a second piece of timber sitting underneath the 3 pots near the back, which they're also screwed onto. The whole thing is a rigid unit.
At the front I've cut holes in play mats and screwed them to the pot rims. There's probably no need for this but it helps keep any driving rain out by limiting the opening size.
Lastly I screwed on a piece of folded corrugated metal. The screws point inwards but I've clipped off the sharp points. The fold goes over the back to stop rain coming in any of the plant pot's drainage holes. (I've left the drainage holes open but you could always plug them with silicone or even some spare corks.)
Since the birds' weight (if 3 hop in at once) might be too much and overbalance the unit by pressing down the front, I've also added about an inch and a half of fine gravel underneath the litter in the nestboxes. This tends to weigh the unit down and help stabilise it, given that it's fairly light otherwise. It also helps flatten the curved 'floor' of each nestbox so the birds are more comfortable when they have to stand up (as they do when in the act of passing an egg).
So there it is, an hour's work, not much to look at but the big white hybrids can't use normal nestboxes because of their stupendous size. They've already started sitting in this one.