this question took me back to 1970 when I was at the Uni.
There was no Autocad or even a PC in the 70 and we used ink to draw our projects.
Usually we got together at the same house and helped each other to finish the drawings before the deadline. We mostly did the lettering for each other, and a lightbox would be handy but we had none.
Therefore we took the glazing frames off the windows and used them as lightboxes, positioning it as a table placing an acrobat lamp beneath.
I remember once it was so cold that we had to nail blankets to close the window openings.
now it seems so easy to build one.
I would make it 120x90 cm. so that it will be sufficient for all papersizes plus a bit of space at the edges. The depth should be at least 20 cm to provide adequate ventilation.
If you know about carpentry and have the tools do it yourself.
Otherwise find a carpenter and get the frame made.
There should be a lot of openings on the frame for ventilation. Ask for 25 mm diameter holes drilled on the frame at about 20 cm intervals or maybe bigger holes at the sides. If not sufficient you can drill more later.
Tell him to provide a groove along the top edges to fix 5 mm thick glass. You also need to close the bottom with 6 mm ply. (insulation type with one side covered with foil which should face inside.)
Purchase 4x60 cm flourescent tubes and fix them on the bottom ply. I would place them paralel to the short edges with 20 cm intervals. 20W should be sufficient but it really depends on the tint of the glass so be careful. Dont go for very powerfull ones because the heat will be unbearable.
Fix the glass top which you can purchase ready cut from a glazer.
If you're keen to do a perfect job, you can also fix an on-off switch and a socket to connect to the mains.
tint of glass/W oftubes/ventilation holes
these are proportional to each other.