Fill entire bottom of stand with custom sump?

[syncro]

Any reasons why not?

There will be a sump chamber for an ATO reservior and the ATO pump (aqualifter) will be dry mounted above. There will be a few inches between the front of the sump and the cabinet doors so there will be some room for using magnetic suction cups and other small devices to hang off the front. Otherwise the sump will extend the full width and all the way to the back of the cabinet.