I guess this is the only to attempt to score LH F return:
Book 2 journey in 2 one way instead of 1 return.
Pick the route that is the least busy and off peak and try to see if you can get LH F 3 days before departure and at the same time try to see if you can get LH F back. It will cost more miles and the only way to use the 241 is 2 people travelling which also more difficult to score 2 F seats than 1.
Those vouchers seem troublesome to use. As simpleaviation said the difference of F and C are not that huge that they will take the extra step to go for it.