A number of them were brick or had brick formers that were left in place.
For the brick built ones, I did think about splitting the slices that make up the model into two sets (odds and evens) and taking a razor saw to make the vertical mortar lines. Put them back in order and you would have alternating courses. The bricks would be out of scale, but easier than cutting up brick sheet or sculpting it in Milliput etc.
I have a photograph of a brick type 24 somewhere, showing the rear protection wall. The only type 22 I know of was brick as well (they varied by location). The adjacent type 27 (hex or octagonal) is concrete or concrete faced (I have not seen them in 20 odd years).
I might have a photograph of a type 25 from Swanage. The slices might need beveling to get the full Armco effect.