There are a number of sources available for lumber loads for flat cars in N Scale. Something I have not liked about most of them is that the core is made of Styrofoam or soft balsa wood. The corners tend to get rounded when wrapping the paper covering over them which to me ruins the effect. Back 20 or more years ago I decided to make my own wrapped lumber loads for the few of these type of cars that I had. I used actual wood for the core, cut and sanded carefully to retain the sharp edges and corners. A photo of one of my Red Caboose 72 foot cars with one of these loads is shown below.
Of course the disadvantage of using wood as the core is the weight. Since I only had 3 of these cars it did not matter too much but recently I acquired several more center beams and now also have a number of bulkhead flats that I would like to have loads for. So with a goal of creating a lightweight core for these loads I purchased some Evergreen styrene square and rectangle tubing. They were cut to length to fit inside the bulkheads of the Micro-Trains 60ft center beam flat car. This combination measures just over 8 scale feet tall.
To make these loads easily removable but secure when installed, I used of some small 6mm x 3mm magnets in the middle inside the rectangular tube. I marked the center of the rectangular tube and also marked a rod to measure the distance between the edge of the tube and where the magnet should be inside the tube. The magnets are oriented so that for each pair of loads the magnets will pull them together through the center beam.
The magnet is positioned inside the tube with the aid of the rod then another magnet is placed on the outside to hold it in place. Then the rod is removed and some adhesive is added inside the tube to hold the magnet. The magnets are place in the proper polarity so that they will pull together once placed on the rail car.
Stick lumber such as 2 x 4, 2 x 6, etc. is packaged in plastic wrap and arranged so that the package is close to 4 feet across. Of course a stack of any 4 x 8 sheet material would also be 4 feet across. The .250 inch width of the styrene tube was just under 4 feet so a sheet of .020 thick styrene was attached to each side to make up the difference.
I make the graphics for the wraps using Microsoft Visio. Some experimenting was done to get just the right size. The graphics for many lumber products can be found on the internet and added to the design. I use Avery stick glue to attach the wrap to the styrene tube.
The weight of a pair of finished loads for the 60ft car is 14 grams. Much of that weight is in the magnets so if weight is a primary concern the magnets could be left out and the loads could be mounted with adhesive as is normally done.
The magnets in the pair of loads hold the loads in place against each other through the center beam but can easily be removed to allow for empty running. This will work with either open or opera window type center beams.
