If you want it to hover in the air, you can set the "Start Asleep" flag under the physbox flags tab. You can activate it with an input from a func_trigger placed beneath it if you wish it to crush a player that passes beneath it. I believe objects that start asleep will be activated by being shot as well, this is how the paintings in the source version of cs_office are handled.

You could also use the "Motion Disabled" flag, with a similar triggering method. The difference between motion disabled and start asleep is that objects with their motion disabled will never move until motion is enabled, unless you also set the "Enable Motion on Physcannon Grab" flag.

If you just want to make it very light, you could follow Rof's suggestion and give it a very small Mass Scale value.

if you're after making a ship of some sort you'll have to pay attention to the hull shape, i made one the other day...tipped and all that shyt so i hunted down some pics on google n found a hull shape that worked, might acctually be common knowledge and i was bein thick as fook, but it works now, make the hull more rounded and wide, if you are making a ship, keep the polys down cos it lags, i'm working on making the actual ship a sort of funcbrush non clip model parented to a playerclip "simplified" physbox model of the ship cos atm its still a bit laggy, althought i've toned it down a bit by parenting funcbrush and telling it not to clip, and making playerclip boxes to block it off. ne ways it works i'll post a screenshot when i get back from college. ne ways look for some pics of hulls from galleons n stuff n model em on that, s'how i got my ship to work. also stairs are a ball ache to run up when ships swaying n tipping n stuff, best to make a playerclip wedge or somert n set the brush for the stairs as noclip n parent as needed sorta thing. ne ways soz if what i've wrotes all garbbled i'm kinda in a rush gotta get back to lecture