Tank repair complete. I will just lay it out here from inside to outside to be sure:
1. Nut
2. White plastic washer (hard surface)
3. Overflow box wall
4. Tank glass
5. 60mm gaskets (I haven't cut them about for fear of messing them up, they are amazingly hard to obtain on their own)
6. New acrylic place (6mm thick, 45mm holes, it is a perfect match to the overflow box)
7. Original (smaller) bulkhead gaskets - but smaller than the diameter of the flange which I believe is what is desired
8. Background print with holes large enough for the flange to go entirely through
9. Flange
10. Pipework
I have to say I didn't centerise the 60mm gaskets particularly well, it was hard to see with all the stuff, and I haven't used any silicone at all (yet). Nonetheless, although I see drips coming down between the overflow box and inside tank wall, which I frankly don't care about at all, the outside is 100% dry with no leaks, which is surprising considering that there is only 1.5mm - 2mm leeway when the 60mm gaskets are perfectly centerised.
I have tightened the bulkheads only as tight as they will go with my hand at this point (I'm very weak, even for my size, due to health).
The overflow box is perfectly level.
So... have I done it correctly? Any changes I should make. I've turned the RO filter back on now so I can finish filling the tank.