Putting channels in the board is definitely on the radar for board #3 but I'm still working out the best approach.
The approaches that I'm most excited about at the moment are the double bagging idea ( a full bag around the board and then half bag to clamp the bag onto the rocker table) and the idea of building the board upside down.
The double bagging idea, I think, would work for boards where the majority of the surface area of the bottom side of the board is not channelled. My thinking here is that area that does not have channels on it will be be in contact with the rocker table surface and so the entire force due to the vacuum on the upper surface will need to be supported by the non-channeled area. This means that the pressure on the the non-channeled areas will be subject to multiple atmospheres of pressure rather than just c. 75% of an atmosphere when the entire bottom surface is in contact with the rocker table.
The resin that I work with says that you should be careful when applying pressures greater than 1/2 an atmosphere as it can cause the lower molecular weight components to separate from the resin mixture and weaken the final product. In the case the pressure at the contact points would be many atomspheres pressure and , at least for foam cores, might be enough to squash the peaks in the channels.
One possible thing that might help ( thinking out loud here) is to build the core up out of several layers of material and do it on the rocker table so that the core already has much of the shape held in place by the internal stresses of the core material layers before you try to laminate it. This would reduce the clamping pressure by the second (1/2) bag if using the double bagging approach. Alternatively, maybe there is a way to use steam and pressure to put most of the rocker and concave in the board prior to laminating.
In production boards they get around this by having 3-d moulds for the top and bottom surface that match perfectly with the contours of the board so that the pressure is distributed evenly. Other than being able keep the pieces cut out from the channels to use to create the 3-d surface on the rocker table I can't really see how achieve this. Maybe it would be possible to make up pieces close to the size and shape of the channels and cover it with compressible material so that under pressure the load could be distributed more evenly. Perhaps the pieces could be cut from PVC foam and use the channels themselves with sandpaper laid in them to then shape to PVC foam to exactly match the channel channels.....
The other idea of building the board inverted (upper deck on the rocker table) has some real appeal. It would put a pristine finish on the upper side of the board straight off the rocker table. It would allow narrower boards to be built as the step down from the top layer to the typically recessed rail reduces the width available for the footpads to sit without there being overhang and it would allow the vacuum bagging to be used to apply a uniform pressure over the contoured bottom side of the board.
The downside is that contouring on the top would have to be very limited unless 3-d mould 'inserts' could be created and it would mean that it may not be possible to use the plexiglass as the rocker material as it would be difficult to get the plexiglass to bend into the concave of the table surface. However, alternate surfaces might work. MDF painted with high temp paint and clearcote might work as might self-adhesive plastic film (mylar for example) stuck to the surface. Using thin metal sheet at the table surface might also be an option. Thin sheet metal might also be good ( I'm guessing) for using in heat curing although I wonder if it might buckle?
My production board CrazyFly raptor has no contouring on the top surface and I really like look of it so for me this upside down idea is pretty attractive.