Flat Roof - Insulation - Joist spacing

G'day this is my first post but I have been lurking for a while and I am grateful for the good advice. :)
I have decided it is time to replace my flat Clip Lok roof (built in 1964).
At the same time, I want to install thermal insulation.
I have set a target of at least R4 and now realise even the thickest anticon blanket won't achieve that.

I am getting quotes but some of the roofers don't seem too interested in insulation other than anticon.
But I think one guy gave me good advice when he said that only batts would achieve the standard I want.

Then the last one to quote suggested that it would be most cost effective if I buy the batts and shove them in myself after he removes the old Clip Lok.
Of course new sarking or blanket of some sort is still required.

None of these guys actually looked at the spacing of the joists.

The joist spacings alternate between 600mm and 700mm because of the way the timbers overlap.
The most common batt width is 580 mm. So if I use that width I will have gaps.

The best idea I can think of is to buy 580mm R4 glass batts (cheaper) and tear up poly batts (more exy but easier to work with) to fill the 20mm and 120 mm gaps.

You guys must have some better ideas. Are there other widths available? Can I get rolls cut to 600mm and 700mm? :(


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