The issue is venting. Trim kits are made to vent the MW out the front so they don't overheat.

Yes, so if the Breville is vented the same way, should it be safe to use, just as the Panasonic is?

Well, it depends - typically a trim kit is made for a specific model MW. The trim kit is kind of a box that the MW sits in so that if the MW vents on the sides or the bottom, the heat is directed out the front of the MW.

If Breville doesn't have a specific trim kit, you might be able to get one from microtrim.com that makes custom trim kits.

Do they still make the same Panasonic MW as you have now? That would be the easiest thing to do.

No they don't make the same microwave. The replacement, which is very similar, is actually more different in size than the Breville. The vent piece that sat on the Panasonic to direct the air from the vent to the front, does not quite fit the Breville, but I could modify it. I am attaching a picture of the set up now, sans grill. I suspect that the air flow would go out of the vent on the left side, rise to the top and dissipate out of the front anyway. That is, the vent helps, but with enough front venting I am hoping that the air would just go out the front anyway.

I have had my countertop panasonic 1.2 cf mw sitting in an old built in mw cavity is almost the same spacing that you have above for three years now without problems. I was worried at first because the instructions call for 3 inches clearance on both sides, and I only have 1.5-2 inches, but I do have 3 inches at the top where it only calls for one inch, so I think that as along as the total space around the MW is sufficient, that is probably good enough for venting.

That looks nice! I love all my Breville appliances!

What size is the trim kit? style #? I love breville and would like to use it built in also!

Any follow up to what trim kit was used for the breville?

It was the trim kit for the Panasonic. I can't remember the model, but just measure and see if you can find one that will work. In the picture, you can see that only the top and bottom is used. It comes with sides, but I did not use them and instead, made it from wood and glued it together. It looks a bit of a mess from the back, but from the front, nice and clean. It's held on with magnets. Functionally, the Breville has had no issues and is working fine in this cabinet.

What size is cabinet w, and what is allotted ht and Depth for the MW w trim kit?

I have the exact same issue with my Panasonic dying and no replacements from Panasonic that fit my trim kit. The Breville sounds like a great microwav and I’m thinking of doing the same as you. It’s three years later, is your microwave still doing fine in the cabinet?

I don’t have the same microwave, but have had the same set up as with the Breville, and so far no problems ( it is 5 years now but used in a vacation home, so not full time)