The seal on my sunroof compresses against the metal roof of the Bongo when the roof is fully closed. The part that overlaps the roof glass is hard rubber and the sides which compress against the metal are a fairly rigid foam. There is a join at the front with a small gap, but that closes as the seal compresses.
It might be posible for someone who knows sunroofs to replace it with a standard rubber seal bent round the corners with heat.
If the roof is not shut fully the drainage channels on mine cope with average rain, but not the sort of upturned bucket showers that have been regular this summer. That is how I got the nicotine stains back on the headlining.
It is lovely sunroof when working correctly. Well worth a bit of effort to resurrect. As suggested earlier the converters must remove sunroofs to put on aftermarket roofs. Also they must use something to seal round the opening to the new roof.
Short term closing the roof as tightly as possible and sealing it with silicone sealant is probably the best way to go. In the long term looking round converters and breakers for a new seal would be best.
On a Land Rover Discovery I fixed a leaking sunroof with a bead of silicon sealent on the actual roof where the rubber seal pushed against. Not sure if this would work with the Bongo as the roof glass rises from below.
Just curiosity. This sounds a catastrophic failure, was it imported in this condition? Or did the previous owner just drive around on a wetsuit?
The BewilderBeast - V6 Mean Green Tintop with LPG and 321 Away Montague Conversion