We have been casting around for a method, suitable for children, of making nest concaves. The answer could well be Modroc - plaster of Paris bandage: it is cheap, simple and quick, and the resulting concave looks ideal for Swifts.
Place the concave former in the centre of the food container, then cut two broad strips of Modroc, soaked in water across the centre. Then cut narrower strips (say 50mm wide) for the sides, building up the rim until it is at least as high as the concave former. Level off the edge of the rim.
A few minutes later you can turn the concave out, remove the former and then start the next one.
Cambridge Swift Tower.