The infill in the base is indeed grille material from ebay - powdercoated aluminium hex shape which is easy to cut to shape and allows airflow
I need to fill in the top of the base fairly soon as I've discovered the power supply fan blows straight on to the underside of the bed at Y=0 which means it takes forever to heat up! I need to create a couple of flat panels and then print up a duct to direct the airflow backwards and away from the bed.
I had the top bearings fitted for a while, but like most of the other mods they made print quality worse by inducing Z-wobble. The reason Creality left the lead screws unconstrained at the top is because they are not perfectly straight. By allowing them to move around, the guide wheels on the X carriage constrain the Z axis keeping it nice and smooth. When you fix the top of the lead screws, any bends in them will push the X carriage around and make the print less accurate. I quickly concluded that Creality left them loose for a reason. Many other printers are exactly the same.
The only time you'd want to constrain the top of the lead screws is if you have perfectly dead-on straight ones, then you won't get any Z wobble. I'm contemplating a further modification to remove the two Z motors and replace them with a single one in the base, driving two aftermarket lead screws via a belt. This will eliminate the vexing flaw with the CR-10 where the two Z screws get out of sync and tilt the X carriage, and by buying good quality lead screws I can put bearings top and bottom without inducing a wobble.
In the meantime I have a plan to print up an indicator arrow with which I can align the slot in the Z motor coupler, then I have a visual indication that the Z motors are in sync before I start a print.