Generally, you'd order the trim kit for a 30 inch cabinet. What microwave are you getting? You will need to make sure that your cut out dimensions will work for your microwave and trim kit (check the specs). For instance, the KA countertop microwave requires an opening cut-out height of 17 inches in order to use the trim kit, and your opening height is only 15 1/4. If your opening is too big for your trim kit, you can always add filler pieces to make it smaller, but if your opening is too small, you'd have to go with a different microwave/trim kit (I think). I have the same situtation with my base cabinet microwave opening and can't decide what to do there.

Sharp hasn't changed the shape of their built in microwaves since the early 90's and hopefully won't anytime soon. The R5 series and matching trim kits would be the model to look for.

There are plenty of micros on the market that are less that 16" tall. You can get a custom trim kit any size you want from Micro-Trim in Cali., or have the cabinetmaker fashion one from wood. The Panasonic linked below would work. This Sharp will also fit 16" and is perfect for micro aplications in base cabs like yours. Unfortunately neither are convection units. http://www.homeclick.com/web/catalog/product_detail.aspx?pid=268462 Here is a link that might be useful: Panasonic micro under 16

We did the semi-built-in hybrid similar to what I think cathy725 is describing. We backed into that by accident. Were planning on a built-in and the cabinet is made to the specs for a panasonic built-in, but a month prior to our reno -- per Murphy's law -- our microwave broke. So we got a $99 kenmore to tide us over. Then when we realized this microwave would fit the space nicely and the cabinet has the required clearances for it (the microwave is smallish), we decided it would be better to do the semi built-in option. So we asked our cabinet maker to put a strip of molding at the top to give it the appearance of being more built in, but the interior dimensions are actually larger. We figure this gives us the best of both worlds. Lower up front cost by reusing our existing micro, ease of replacement if it breaks, and the cabinet is still sized for a built in if we ever decide to change our minds on that. But given our experience so far in the 14 months we've had this, I don't see ever switching back to a built in now. Here's what ours looks like. A bit hard to see the molding strip that makes it appear more fit-to-size, but the interior cavity goes up to the bottom of the frameless cabinet door above the cutout. Please forgive this grainy pic that makes the mw look horrible, it's not a high enough photo resolution to be blown up that big, but hopefully it gets the point across: