Lots of posts about how glue doesn't work, so I thought I'd post my fix. FYI the parts are marked as PP (polypropylene). PP is what gets used in things like tupperware because it is very resistant to chemicals. That also means it's very resistant to most adhesives.

You'll notice that there is a piece of wire that is embedded in the joint. Honda clamps the two haves together and then heats the wire (probably with radio waves) to melt the joint together. We don't have an RF generator, but we can heat that wire with a battery charger. The trick is not burning through the plastic.

You'll need a battery charger on a switch (we only need it on for a few seconds) and some alligator clips that are small enough to connect to the wire in the joint. There is a loop of wire in the center of the headrest. You'll need to cut it along the bottom and bend the two ends out to that it will make a circuit. You also may need to expose the ends of the long wire that runs along the outside.

Before you heat the wires you need everything to be clamped together. I put the frame in the vise to hold it still and used a bunch of small bungie cords to hold the plastic. Everything should be firmly clamped. The entire joint needs to be closed. It needs to stay calmped throughout the heating/cooling of the wire. Make sure you've routed the seatbelt properly.

I heated the center hole wire first. Charger was on the 10A setting. Probably less than 5-10 seconds of on time. The outside of the joint will be hot to the touch but not melting. A little bit of smoke where the wire exited the channel as that piece of wire will get hotter. Then move to the outer wire. I think it might have been a few seconds more heating time, maybe 10-15 seconds.

Let cool and you've fixed your roll bar cover.