Fill entire bottom of stand with custom sump?
 

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.

nothing wrong with it - alot of the cadlights setup are like that with the sump taking up the entire bottom of the stand. Just make sure you measure right so the sump will actually fit into the stand when you first go to put it in.

sump
 

mine goes the entire length except for where my return pump and wavemaker is

It depends on you hardware preference. If you plan to eventually get a recirc skimmer, external reactors, or a two part dosing setup, you will have to leave some room.

I've run out of room under my tank, with pumps, CO2, etc. But still happy my sump takes as much room as it does. The bigger sump makes everything a lot easier, other accessories can hang outside the stand if needed.

Thanks for the feedback phi delt reefer, monocus, medhatreefguy and Lampshade. I'll go with a full size sump for now.