Add more filters you already have. If you add all three colors, Y+M+C, you get neutral density. So use 50Y+50M+50C plus whatever your pack is. If that isn't enough add the 40 filters for 90 of each. Of course the filters are probably not perfect so the rest of your pack may have to be varied a little, but this approach will work.

This is sure a far cry from the first color prints I made which were from slides on Kodak type 1993. (Not the date - it was in the 80s.) Exposures of 90 seconds were not unheard of.