Like the old ERF Snorkel crew cab, why not just turn around a set of factory front doors and - with a few minor modifications - use them as rear doors for the crew cab. Then add a small C pillar window so the rear seat occupants can see out a bit more easily.
The modified roof was intended to increase headroom, which was a bit constricted on those Volvos like the London examples which retained the original truck cab roof height. Looks like it was molded to accept two beacons rather than the light bar on this unit, though.
Added by Rob on 24 July 2022.