You STILL owe it to yourself to go see a reputable dealer of helmets. That’s really the best/only way. If you ever get to Texas, I’ll show you my collection of “mail order” helmets from 20 years ago that I bought for a good deal and never fit or felt good.
Another point; don’t get hung up on what the size is. Only women care if they are a size 2 or a 6. Who cares? If you think you’re a large, start from there. I wear a Large in a Shoei and an XL in an Arai. I’ve worn a XXL in a Bieffe…doesn’t matter.
As another poster pointed out, many helmets have customizable interiors from a sizing stand point. Can’t give you an all inclusive list, but you better helmets usually do.
I would not recommend modifying a helmet to you in any way. There is a helmet that fits and fits well for you. A helmet mod for comfort is folly…and ill advised. Can you get away with a bandana or skull cap? Sure, that should pose an issue. But, for instance, I had a guy in one of my classes who wore a baseball cap under his lid. I asked how that metal button was going to feel driving through the top of his skull in a crash…he hadn’t that of that…
As always, hope that helps.