So i'm not sure, but the "Cannot Wear Clothes" gene seems to cause some issues with warcaskets. Mostly bringing it up as there's clearly some overlap in "big creatures can use warcasket weapons" and vise versa, so it might be worth adding a note at least about it.

Before going in the Warcasket Customization menu gets put into this bugged state where your choices don't seem to properly reflect on the model, and color changes are slow to update.

When i applied it, what happened was it effectively put the pawn in this weird broken state. It ate the materials to make it, the warcasket trait was not applied, and a helmet was put on.
Now their other slots show no armor/equipment, but they're now unable to wear armor (though they could before). I can confirm they're not getting any stats from it either, minus the helmet.

Using character editor i was able to just undress the pawn to remove it, and then i had to play with the genes. Removing the no clothes gene and trying again seemed to fix it and i was able to build the warcasket around them. I made sure none of their other genes were causing issues.

Now i assume this is because the warcasket occupies all layers including skin and therefore is hitting the "no clothes" part of that.

Mostly would be nice to add a check for the "Entomb in Warcasket" option to check if they have that gene and disable the option. Otherwise maybe a quick note about it.