I work with harness leathers up to 16-18 oz some on the smaller ones & that is plenty stiff. Some of the larger trunks are made with Palonia & Poplar frames which is as close as I can get to what the old original trunks were made of. The old trunks coming from England was made from Deal.
I thing there are some detail pics on my facebook page. Try Two Fools Leather Goods on facebook.