My wife’s using something smaller than that. She gets them at Costco, that’s all I can say.
It’s smaller in diameter than my ONE, but larger than my Little Guy, and therefore way larger than my Mighty Flea.
You can also just get a hole borer for drilling large holes and just use a size that seems to work for you. But for cutting through foam, it seems excessive, expensive and too single-purposeful. You could also make a pattern by first starting with your under-sized throw of choice, then use that to make your design that you then transfer to your foam. Whatever you use, make sure the blade is very thin and long. I like non-serrated edges since it makes less mess but requires a bit more patience but can result in cleaner cuts. Some people like serrated blade edges and do more sawing, but I find it requires more clean-up and the edges for me tend to fray over time. Do notice that many cases are using star or some other design that cuts in and out. This is to facilitate holding lots of different sizes securely without being set on any one size. The cut-ins are exaggerated in order to make this work.
Another idea, although it’s expensive, is a Pelican case. These are filled with die-cut foam, allowing you to remove bits to make almost whatever shape you want. Think reverse Lego. You may not get an exact fit, but you can get something close. The problem is that sometimes you have to take out too much so you end up with too large of an area. Before you remove blocks, put the item down to see what you SHOULD and SHOULD NOT remove. I suggest having a silver sharpie on hand to help in this process. In fact, before you begin, you should lay out everything onto the foam before you remove anything. Many are large enough where you can do multiple layers on the same case half depending on your layer thickness. Some can also hold a layer on the top as well.