A bank offers their customers a one-year investment that pays out the return of the FTSE100 index if the FTSE is up 5% or more, but their total return is capped at 15%. If the FTSE100 is down, or up by less than 5% the customer will get their initial investment back in full.

How might such an investment product be structured? Why might it be difficult to issue such a product in todays market environment?