yes, the inner parts were wobbly during construction, because they are only 5mm wood pinned together with very small nails with a nailgun.
we pretty much just figured out how to build it exactly while we were doing it.
that is why you see all those little woodblocks on the inside, they helped a lot in stabilizing the inner to the outer wood. one plan we ditched on the way was to make little cuts into the outer shell, so the inner wood could have been slid into this cuts and not slide around, but we didn´t have the right size of "carver?".
anyways if someone would like to build that design i´d recommend not to use to thin of a material but to use at least also 1cm for the inner construction. the diffusor would than be a little more heavy, but it will also be much easier to build. and it doesn´t matter that much if you need two or four persons to carry it, because that is mostly done only once.