Fair enough...
The easy option is to just get the 25mm aluminium plate, mill it to size and drill the array of holes in it. Alternatively use the same thickness, but mill slots into it then add strips on top (something like 1.1/4"x3/8") so that you end up with T-slots which makes clamping much easier. In addition this means you can skim the bed flat a few times, or accidently make marks in it without worrying too much since the strips can simply be replaced which wont cost much.