I almost forgot (remembered when looking through my pics), other than very few exceptions, there is no such thing as a flat building board. I absolutely cannot stand to start a build with a board that isn't perfectly flat. Errors are going to creep into the build, so you have to start off perfectly flat. This is how:

1. Find a strong table that is as flat as possible. The building surface will be a 2’x4’ suspended ceiling panel from Home Depot or somewhere.
2. Place a long ruler lengthwise and widthwise over the table to find where it sags – they all sag.
3. Use 1/8”, 3/32”, 1/16” balsa scraps under the celling panel to level it out.
4. Once you get close to perfect, take scrap 8 ½”x11” paper to get it perfect.

The pics below show my board being taken off the table once it was no longer needed.

Quick note about glues (not epoxy). I think I've tried most of them and found that Titebond Original works best for me. Titebond III is "rubbery". I sparingly use thin, medium and thick CA, mostly in gluing in ribs. I also like my 5 minute epoxy, dabbing it here and there (forward of the balance point). Ratio of glue use is something like Titebond - 85+%, CA - 5 to 10% and epoxy 5 to 10%.