Real life has kept me busy since New Year's, but at last I can show some progress again. I wish to portray the ship carrying lumber like she did much of the time during WWII. Not very exciting, not terribly war-winning even, but necessary.
Now, lumber is not the heaviest of cargos, so the ship would max out on internal volume long before maxing out on weight. One often sees such ships carrying lumber as deck-cargo, stacked high and wide. Here are a couple of examples:

"Anna", a small Norwegian lumber-freighter not very different from "Norse Lady", carrying neutrality markings (WW I? Unknown.) Note that lumber is stacked absolutely everywhere, even blocking the wing passages below the bridge!

"Trollnes", a slightly larger ship than "Norse Lady", again carrying a deck cargo of lumber. I am guessing this is WW II era, plusminus a few years. She is not as madly overloaded as "Anna". Note the athwartships (chain?) straps securing the lumber.
These are just two examples, there are many more out there. Of note is the tendency to fill in the well-decks while leaving the cargo booms free (just) to operate. A small "well" is left open around the masts and winches. Sometimes a gangway of planks runs on top of the lumber piles, connecting forecastle/bridge/stern. It is quite common to see temporary battens supporting the lumber piles set up along the ship's sides, again made from lumber. Cannibalized from the load perhaps?
Now, the first thing I did was to use cardboard and mark out the available space for my cargo in the well-decks:

I need to leave room around the masts, for stairs and ventilators at each corner of the deck, and at the sides for the mast's shrouds. The shrouds do complicate things!

Cardboard templates. I then cut out the same shapes from plasticard, the plastic rests on top of the cargo hatches so have to be extended downwards to deck level and up to the desired height.

Nearly completed deck cargo "pallets" - I have made them so that they can be removed from the model for painting and installed as units later. The bow pallet, left, shows an all-plank cargo, supported at the ship's sides by wooden battens. The stern pallet, right, shows a more mixed cargo - some planks at the sides, logs and large square timber balks midships. Here I have added the gangway mentioned above to connect bridge and sterncastle. A similar one will be added to the bow pallet.

The model with cargo pallets in place. The masts will be installed at the same time as the finished, painted pallets - the shrouds run through openings in the deck cargo so it all has to be done rather carefully.