I wound up asking the lfs where the tank MFG puts the bottom panel for the stand and decided to raise it off of the floor.
I did this by putting braces of 4" widths across the bottom of the tank, 3/4" plywood - 3/4" up from the bottom of the stand and inside the framing. Then I set the 3/4" plywood floor panel on top of the braces (everything was glued and pocket-screwed).
Also, I put a 3/4" plywood deck on the top of the stand just in case there were any imperfections in the 2x8 joints.
Question is, with the 45* braces at every corner like I've installed, is there still a need to skin the back and/or sides? If so, what thickness of plywood is recommended?
What did I learn?
Pick the straightest, non-warped boards you can.
Use the Kreg jig or similar pocket-hole system
Glue every joint
Keep wet paper towels handy for glue clean up
Build it stronger than you think you'll need (peace of mind)
Use (or find someone's to use) a table saw to rip 1/8" from both sides of your stock to remove the rounded corners to make the joints nicer/stronger.
Have a lot of clamps on hand
If you are going on carpet - raise the floor of the stand at least 3/4" off the bottom of the stand and place it inside the 2x framing.
Glue and screw every joint.
Make the "box" for the top separately, make sure the joints are rock solid and planar (flat) and put it on the legs as one piece. Putting the top together one piece at a time and installing each piece separately on the legs makes it very difficult to keep the top joints flat.
A plywood top is nice and strengthens your stand while making up for "minor" imperfections in your 2x top.
Use only glue, no mechanical fasteners, to secure your top to the stand. Make sure to clamp the top down while the glue dries.
Foam is only needed for acrylic tanks and glass tanks that don't have bottom trim.
If you have a glass tank and it has plastic trim around the bottom, you don't need to use foam - it's actually a bad idea to use it when combined with a glass tank that has the plastic trim on the bottom.
Feel free to add or change this list, it's simply what I gleaned from my research when building RocketEngineer's design.
Thank you RocketEngineer!
Here are my photos:
I still have to remove the clamps and route the overhanging top flush with the stand.