I didn't try silicone because I didn't think it would have the strength.

Two part epoxy (Araldite) is incredibly strong if mixed and cured correctly. I mixed it up and applied to the half with the groove then fitted the two pieces together. I had masked off the edges so any excess went on tape not the outer plastic.

It is important to support the roll bar so that it doesn't lever the two pieces apart. I used gaffer tape stretched tight around the top, sides and bottom to hold the pieces together during curing and left for 24 hours.

It has held nicely now for many months. That said, if someone was offering to buy me a new one I would certainly take that option.