No thats just a frame and yes I did do it with 2x4's but I've been thinking really hard on this tonight and probably will end up using hardwoods.
Regarding the frame, will you be using 2x4s on the side units as well? I think it'll be overkill, and you'd be fine with using vaneered presswood to make those side pieces.
As far as the braces go, yeah I am wanting to have something there for some sort of support but still enough access to the sump. The overall cabinet will be completely covered, but I have not been successful covering this frame as of yet with wood products within the program even though it does have a layering capability.
I wouldn't add the braces either. That thing looks like it can hold a tank! lol. Little piece of advice, for what it's worth, if you can, laminate the interior cabinet of the sump with an arborite. or use a presswood with a melamine interior. It'll make cleaning much easier, and evaporation from the sump won't get into the wood.