Well I have come to the conclusion that there was never a cover for the AD Suburbans Tool Box Hole.
My reasoning is, if you look at the Panel Truck riser Picture 1 and Picture 2
Look at the outline of the hole openings.
GM stamped a 3/8 inch deep recessed lip into the perimeter of the Hole.
This is done so the Cover is relatively flush with the top of the riser or seat platform.
Now if you look at the Suburban Riser Picture 3 - (Click on Pic to enlarge)
There is NO 3/8 inch recessed lip stamped into the perimeter of the hole.
The opening looks finished with a nice rolled edge.

HERE are the Dimensions of the Panel Truck Holes.
The lip is recessed 3/8" deep, the width of the lip varies averaging about 1" wide.

I was thinking if you really didn't like the Suburban opening uncovered.
You could find a donor panel truck riser and cut out the tool box opening and butt weld it into the Suburban riser.
But then it wouldn't be stock.

Tool Box Cover Hole Photo Album