From what I understand you don't just need to seal it up. I haven't boost prepped a nikki, but I've worked with boost prepped webers and dellortos.
It does need to be sealed, but it also needs to behave as if it is not pressurized compared to the outside air pressure.
If you could develop a theoretical giant hat that covered the whole carb, fuel tank, fuel pump and lines, you wouldn't need to do anything different at all to the carb besides jetting.
Now you can shrink the theoretical hat to cover just the entire carb when you add a rising rate fuel pressure regulator (RRFPR). Now the RRFPR doesn't "push" in extra fuel to enrich the mixture, it just makes the fuel come in like normal. so the system behaves like their is no pressurized air. the boost would make the fuel flow slow until eventually it pushed it backwards, the RRFPR simply keeps that from happening, keeping flow like normal. The proper jetting is what enriches the mixture.
You need the look at the carb and remember that it needs to "think" it's not pressurized. It needs to "think" that some how when you step on the throttle, the whole vehicle is magically going into an area where the air is thicker and more pressurized then just one atmosphere.
For example if the jets are sealed off from the boost pressure, like in a dellorto, JUST SEALING UP EVERYTHING WILL NOT WORK. There needs to be a path for the boost pressure to reach them so it will operate correctly.
I believe this MIGHT be the case with the nikki. The air horn (top piece) looks like it seals off the venturis from the jets. So you might need to drill in the right places to let the fuel bowl and jets "see" boost, so they "think" that the compressed air is everywhere, like a beautiful, thick, turbocharged atmosphere.
This has me curious, so I'm gonna look at my old nikkis. But if anyone want to take a peek at my boost prepped dellorto and weber IDA, I'll post pics. It might clarify what I stumbled through describing above.