Just to add a few points on water tray depth, I run a 28 mm water depth at around 5 mm under the metal and it is not ideal. I think the extra work became a non starterr. I think the plan was for a water table to cut down on the air pollutants. The other option was to possibly lift the Y axis up by extending the height of the sides to give more clearance. One issue that I seem to remember with Neds first table was that the main x axis is down the middle of the table, so consider the work area gets replaced by slats and the crud will be blown with force downwards and possibly onto the ballscrew. Have a look for some posts from Ned, he started with a very similar looking table to the one you posted and I think gave up on that and changed plans.