I agree with Jpmom that you don't want a MW on top of a double oven - it's just too high. However, although she does not like a MW drawer, many people on this forum adore theirs (note that all MW drawers are made by Sharp, so if you decide to get one, no need to pay up for a different brand name, as it is just a rebadge). Is there no other location in your kitchen? Do you have a pantry where you could keep the MW? There was just a recent thread on this MW placement topic. http://ths.gardenweb.com/discussions/3833699/placement-of-a-microwave?n=42 I am not a fan of the oven/microwave combos, as I think there are too many compromises in both in order to combine them. Just MHO. If you are doing a major kitchen remodel, you might want to re-think the cooktop in the island, mostly because how are you going to exhaust it. This topic has been covered a zillion times on this forum, but the upshot is if you care about good exhaust, you should have your cooktop on a wall, so that the exhaust capabilities are not hampered by trying to exhaust over an island setup. On the other hand, many people are absolutely fine with poor or no exhaust, and perhaps that is you, so never mind.

We have an oven/microwave combo (KitchenAid). If I could redo my kitchen I might do that part differently. I did not know when I designed my kitchen last year that microwave drawers were a thing. If I had known I might have chosen that with a built in oven instead. I do not dislike our oven/microwave however I do have concerns about the microwave breaking. The fan was actually broken when we installed it, and we had to get it repaired. So if something happens to it, I know we will have to pull the entire unit out to get it fixed which is more of a pain than just fixing a microwave. The other issue I have with it is that microwave is a little higher than I would like. We installed the unit 1.5" higher than they recommend (so I could have a 15" drawer below) and since I'm only 5'3" the microwave is a little bit high for me. Even installing it at suggested height would still leave it higher than the ideal. A drawer would be a nicer height most likely. I definitely like having a separate gas rangetop and built in oven. There are many reasons I want them separate, so a range was never an option for me. But I probably won't combine the oven and microwave again if I did another kitchen.

My daughter did what you are planning on doing. I’m not sure her gap was exactly the same size and her ovens were only 24” wide. Since she did not want to remodel her whole kitchen, she stuck with 24” wide replacements. She put the oven on the bottom and used a speed oven/microwave on top. Like hers your microwave will need a shelf to support it. She had a carpenter put in a shelf and a filler piece between the lower oven and the upper. She also needed an electrician to put in an outlet for the microwave/speed oven. Her over the cooktop microwave was replaced with a vent hood Good luck with your project.

Combo units are expensive, and expensive to replace. Microwaves will eventually die. Replacing just a microwave is easy, and inexpensive. Better off getting a double oven, and separate microwave. You can put the microwave on a shelf, and get a kit to make it look built in, or simply install it on a shelf like this: