I'm guessing you don't need help anymore. Unfortunately, the Kitchens Forum is not very busy during Thanksgiving, Christmas, & other year-end holidays (especially since Houzz took over).

If you do still need help (or if others are reading this who need help), price out the cost of a built-in MW (MW + Trim Kit + install) to see if you will really be saving money. If you're putting a cheaper counter MW under the counter, then it will be cheaper, but it's unlikely you will need (or can even use) a trim kit. If you're getting a true built-in MW, check the cost, some are less expensive, others are more expensive than even a MW drawer.

Regardless, are you OK with steeply bending over to use the controls on a "regular" MW under the counter? Non-drawer MWs are really designed for either on or above the counter, not under the counter. Drawer MWs have a control panel that's angled so you don't have to bend over just to read and operate the controls.

In addition, you will be reaching blindly into the MW to get to the hot dish/food - be very, very careful!

If you don't want to spend the money on a drawer, maybe you would be better off planning for either one on the counter or in an oven cabinet with the MW at or above the counter (but not too high since if it's too high that can be a safety issue.)

The size opening will be determined by the specs of your MW and its associated trim kit.

Thank you so much!