safe cheap easy not doable with any accuracy
you could try a 22" saw and go round all edges perhaps 50mm deep then cut all corners at perhaps 45% untill the blade tip disappears----at that point you have no idea what the blade is doing so further cutting is wait and see
Alternatively cut down to 500 mm in width and again go round 50mm deep 'corners again then alternate side perhaps 100mm to bring each side back on line as best you can
i have cut small bits easily where you can see the full blade width quite accurately
I've found I can rip most softer insulated sheets or batting, e.g. Cellotex, Kingspan, mineral wool, sheeps wool, etc with an insulation knife (try Stubai), but the depth limit on mine us about 250mm. Bahco do sell a 500mm (?) insulation saw with a wavy edge which could do it as well (giving maybe 900 to 950mm total width). I have the Bahco, but rarely use it as the Stubai is great for stuff like mineral wool batting. TBH, neither is ideal and the results from either won't be pretty
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