I've been playing with my gearboxes this afternoon and the fireblade 2000 box does fit, but without the rear shock mount attached.
If i dremeled the excess mounting points on the top of the box it could be made to work with the unusual shock mount. I would also have to remove about 5mm of material at the front of the box so it can fit underneath the top deck. The long metal suspension mount could be tricky though, that gets in the way of fitting the gearbox. If i remove some material from the base of the box it would clear ok maybe, or i could use some normal plastic pivot blocks instead and there would be no issue.
Fireblade 2000 wishbones do not fit however, they're slightly too wide for the pivot blocks.
I think the Cougar 2000 gearbox might be a better fit looking at your pictures Jeroen, it seems to have a lot less bulk than the fireblade one.
Thanks for the offer by the way Mick, i might take you up on that if i can't find anything suitable in my parts box
Paul.