The game dislikes negative mass. But I suppose you could clone the blockhead, make it antigrav and not draggable, then put a compositegeom around it.

what do you mean add compositegeom around it?

If you make a CompositeGeom (several rectangles or circles stuck together, essentially) of four small rectangles around the block, then the block will hold it.

If you want an anti-gravity geometry that floats in the air and the goo balls not then you have to find where it says geomonly or soever and change it to true - I did this to my level Goo machine launch zone.

thanks Wiki
but i still think Albino Pokey's suggestion is beter
well.... Albino Pokey's suggestion is a little confusing for me
i start with wiki suggestion

Try opening the "Escape from the World" level from TfR in WooGLE. They have one there.

Or if you're following Wiki's suggestion, make some water or whatever, and give it a big force (>10) upwards. Then set it to "geomonly" if (and only if) you want Goo Balls to sink, but geometry to float.