Becky, nice mainsail cover.
sv strider, I've never seen a boom bag type mainsail cover with two boom zips, and if you made it that way, there will be a gap there on the top where the two center zips begin. However, you could have the sailmaker
or trimmer (or whoever builds that boom bag) sew a flap of whatever canvas
you use that will fold over that zip starting place to protect the sail underneath and velcro to fasten it down. Something else that can help is a zip pull extension, out of stiff plastic, with a similar flap at the aft end to secure it from blowing around. You'd want to be able to secure the "stick" under way, but all it could be figured out how to keep it secured both under way and at anchor
You wrote you're on the Gold Coast, why don't you try talking with some other Cat owners who may be able to point you towards a trimmer or sailmaker
whom you could trust to do a good job. If you haven't already thought of it, it should be designed with copious covers to keep the sun off the zips, they are prone to UV degradation. Also, darker colors absorb more UV than the lighter ones. Finally, specify PTFE thread. Ours is still good after 13 yrs, whereas all the UV 92 polyester thread goes in about 5 years (in our usage, maybe subtropical most of the time, on the order of 3 mo tropical.)