If you can't keep your base on the guy, don't use that position. Go for the knee on the belly or some other position. It's may not be the only way, but it's probably the best way to overcome a physical inability to stabilize. Like any tool, if it doesn't work in a given situation, use something else that does.
I've always been able to keep a low mount with the hooks in on bigger guys, by the way. This might be what you're referring to with the Japanese Mount reference, but I am unfamiliar with that term.
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