Hi,
That's what I've done, built an airbox cut down from the original NS400 one and rebuilt with fibreglass. Building the airbox is ok to do - but I can't build a large airbox because there isn't space, the NS400 frame is a lot wider than the RGV one. The airbox I've built which just squeezes into the RGV frame is a lot smaller than the original NS one.
Racers run open carbs, isn't individual filtering the next best thing to open carbs for top end power? Like the individual filters that people fit to RG500s?
Mat.