I kind of have the same problem.  I have a building board, well about five of them, that I put on the table as needed.  Mine are made of high density particle board, topped with a cieling tile.  That works for me.  My table is also on castors so I can move it as needed, saves having to spin plane around and chance dings, etc.

Every since having a work bench in an attic situated so I could only get to on one side my dream was to have a work bench I could walk all the way around. Now I have 2 and love it. One started life as a fairly well built desk. Top wasn't perfectly flat so I bought a 4 x 8 sheet of 3/4 MDF, cut a 4 x 6 piece for the top. Shimmed it flat using old time cards and covered it with a sheet of wall board.

2nd started life as a metal work bench, 6' x 30", put a piece of hard board on top shimmed as best I could with time cards. On top of that I put a piece of 1/4" glass acquired for free. My mistake with the metal work bench was gluing the hard board down thinking it would stay flat. Over time the metal bench has sagged a tad in the middle. Now I don't have a choice but to shim between the glass and hardboard.

In one of Bob Hunts video magazines (can't remember which one) he demonstrates his method of checking a bench to be sure it's flat. Basically you cut a few 1/8 inch ply squares. Put one in each corner with a string running corner to corner across the bench top, use one of the squares to slip under the string at various locations like a feeler gauge.

You only need a straight flat surface big enought to build the wing on.  Get 2 each 4 ft lengths of 12" wide melamine shelving material from your local home improvement center.    One is your building surface, the other you rip it up into 3" wide strips which you glue and screw to the underside to keep it straight lengthwise.   Its portable,  you can put it on any old table, and work all the way around.  About $15 of materials.
The melamine is a nice smooth white surface which you can draw reference lines on, or cover with your preferred building surface.  Added benefit is the melamine acts like a seal from moisture, which combined with the ribs will keep it straight over time.  Good idea to also seal the edges.

Allan's basically correct in that you only need a straight flat surface big enought to build the wing on but if you have room. It's really nice to have a flat surface you can sit a whole airplane on, comes in handy for bench trimming or chasing a weird trim issue. It allows you to block up the wing at 0-0 and measure everything.

Mine has remained suprisingly stable over time. I do pretty much like Brett describes. It's a maple wood top, 1.25" thick. I check it every couple of builds and use a long sanding block (2x4 piece of rock maple) and 50 grit sand paper to level it up. I use a laser level to check it.

    Highest I ever measured in my bedroom is about 119 degrees, with essentially zero humidity, 109 outside. It was kind of hard to get to sleep that night, and the kitchen floor still felt hot the next morning.
     Brett

That's crazy.  Where do you live ?

Brett did you forget about the blocks of ice in a galvinized tub with a fan blowing over it?  Seen pictures when I worked for Ma Bell in which that is what they did to keep switch room bearable.  Now a days is's new fangled thing called an air conditioner.  Mine in the new shop works when it wants to.  Have fun,  DOC Holliday