Those are some nice pieces, I can see why you want to keep them (looking stock).
It looks as if the joints already have built in diagonal, wooden, reinforcements at the joints. But thanks to wear & tear, these have cracked at the joint.
Knock on wood, they shouldn't be too difficult to replace. Simply make some fine, partial thickness cuts into the frame on both sides of the old reinforcing cross pieces. Using say a fine woodworker's saw, or in a pinch, a hacksaw. And then carefully chisel out the broken bits.
- Ah, & make sure that when you're doing this, the framework is in a supportive jig. Preferably one made out of materials which epoxy wont stick to.
Then once you have the old ones/bits removed, & the joint groove cleaned up, you can manufacture some new reinforcing cross pieces. Making sure to sand them to fit the old joints. And that the final sanding
pattern, if any, matches up with the one on the hatch
Although, if you like, you can get some composite reinforcements, in the form of Tows, or very thing glass, add this into the joint, underneath of the new wooden cross pieces. But be sure to allow for this thickness when sanding
the new wooden reinforcements to fit.
Also, if you like, you can add a thin layer of composite reinforcements to the joint on the underside of the frames also.
And either way, with or without composite reinforcements, make sure to follow the usual precautions when gluing teak. Plus, as already mentioned, things like clear plastic packing tape is good for taping off anywhere that you don't want any epoxy to wind
up. Plus, it's good for holding things together while the glue dries.
PS: For projects of this size, it's easy enough to make your own reinforcement "Tows". Just put on a pair of rubber gloves, to keep your skin's oils, off of the glass or carbon fiber, & "unweave" some heavy cloth at the edges. What you're after is those 3mm-5mm wide bundles of fiber, which are tightly packed together, & all running in the same direction.