We are lucky enough to have a local company with very inexpensive cardboard -- only €1 per single ply sheet that is 1140/940 (mm), €2 for a double ply sheet of the same size.
We then calculated a DOME size that would let us get 2 triangles from each sheet - so we only need (45+30)/2 sheets for the top, plus 14 sheets for the base. This works out to be a 3.9 m DOME, with the A length 79cm.
Total cost of cardboard €66 + VAT = €81.
Given that this is in Ireland, not known for being inexpensive, it would be worth ringing around to a few cardboard suppliers to see if they have this too.
This company do not have an online presence - I found out about them by ringing the National Sculpture Factory, where my colleague Niall is on the board of directors ( https://nationalsculpturefactory.com/) and asking them who they knew of that did cardboard!
Acquiring a digital projector was not an issue for us. My school happened to be in the process of swapping them out of classrooms in favour of large, flat panel displays. It turned out that we already had a suitable half hemispherical mirror in our department, too. Win!
Our Finance team directs we must purchase only from a list of preferred suppliers. Unfortunately, their white-faced, double-walled corrugated cardboard was relatively expensive. Although funding was secured, concern was raised about its durability and the longevity of the project. Our technicians were not granted time to machine the sheet material.
I am grateful to Neill Sanders at Go Stargazing and to Ross Hockham at UK Astronomy for lending us their inflatable domes. With their support, I was still able to share my presentation from the first D.O.M.E. Summer School and to join the British Association of Planetaria community, a veritable cornucopia of knowledge and educational resources.
Many thanks also to Colin Hutcheson from Thinktank, Birmingham Science Museum who has been a fantastic mentor. Colin has inspired me with creative ideas for shows, helping me to develop my presenting skills in his 10-metre planetarium dome in front of an audience of 70 children!