Squaring up HO structures - Model Railroader Magazine

If you're building styrene structures, use the same material for the floors and bracing - much easier with which to work and you can use the same adhesive, creating an even stronger model.  Evergreen sells strip styrene in a variety of shapes and sizes, and any plastics supplier has sheet styrene in 4'x8' sheets - I prefer it .060" thick, but it's available in many thicknesses.  Use a utility knife to cut it into the required sizes. 
This latter material is perfect for making roofs and floors, and for adding foundations to existing structures.  It's also a great material for replacing those walls on kits which will not otherwise be seen on your layout if it's an around-the-room type.  Those surplus walls can be used to enlarge the structure from which they were taken, or kitbashed with similar walls from other structures to create totally new ones. You can also cut strips 1/2" or 1" wide from the large sheet, and by gluing them at 90° to the back side of structure walls, use them as bracing.

Another important consideration when trying to build structures truly square is that the mating corner sufaces are themselves square.  For butt joints, the mating edges need to be square with the bottom of the walls, but also square across their face.  For corner joints in which the mating faces are at 45°, ensure that both are truly 45°.
An easier method is to file or sand these edges to an angle sharper than 45°, then, after ensuring that the side of those edges which will be visible are 90° to the bottom edge, apply solvent-type cement to both mating edges, bringing the pieces together while both surfaces are still soft.  Press them tightly together (some material may ooze out - don't touch it) and then position the assembled pieces within the arms of a suitable machinist's or carpenter's square.  This is not a particularly strong joint, though, so when the cement has cured somewhat, add a full-length of strip styrene to the inside of the corner - .125"x.125", securing it with more solvent cement.  Once the joints have fully-cured, use a sharp, suitable blade to trim away any material which oozed onto the visible surfaces and then touch-up as necessary.

Here's Walthers Vulcan Iron Works doubled in length by substituting .060" sheet styrene for the unseen rear wall.  The same material was used for the roof:

I used a couple of Walthers Waterfront Warehouses to create P&M Languays Pump and Compressor factory, again using plain sheet styrene for the unseen sides, roof, and interior bracing:

However, because the track here isn't parallel to the backdrop/wall, the structure isn't square:

Here's a look at the interior:

The large styrene sheets are also useful for scratchbuilding.  This one is another trapezoid due to the locale, and the doors and windows are leftovers from Languay's:

Wayne