So far this is the best supplier (in the UK) I’ve found. The sheets are pretty good and work out around £1/sheet. Look elsewhere if you like, but the key thing is to get ‘mono’/single ‘flute’ cardboard – i.e. just

Curved Joint

Curved Joint

Concave / Convex Joint

3 Concave & Convex Joint - original edit - img

90 Degree Joint

90 degree joint