this is for vacuum presses only.
I like to press cap skis with just a tiny (2-2.5mm thick) sidewall. To get enough pressure everywhere (especially for the topsheet as it is quite stiff), I put a 3mm board below the ski to give the ski more height in the press. This way I can assure there is enough pressure even close above the edge. Here's a pic:
At the tip and tail sections, the ski is flatter anyway, so I leave the hardboard at its full size, makes fixing it to the mold easier.
When I lay up next time this way, i will make some better pictures.