If it’s a hard case, another option (a bit DIY) is to get mounting hardware from Ortlieb or some other pannier company and mount it to the portfolio. Then it could easily attach to a rear rack. You could set up the mounting brackets however it makes sense based on the riding position/size/etc.
I’d probable do a good quality rear rack, then bolt or strap a piece of exterior plywood thats slightly over the size of the portfolio to it. Then bungee the portfolio to the plywood.
I assume what they are transporting can’t be rolled up? I’ve transported architectural sets before (sometimes several hundred 24×36 sheets) but I’ve always rolled them up and jammed them vertically in a pannier.
If it’s a hard case, another option (a bit DIY) is to get mounting hardware from Ortlieb or some other pannier company and mount it to the portfolio. Then it could easily attach to a rear rack. You could set up the mounting brackets however it makes sense based on the riding position/size/etc.