As pj123 says you need a dedicated film scanner for the best resolution and colour response.....Price and quality of result go hand in hand, but start much more expensive than the high volume selling flatbed.
There was a "Blue Peter" like DIY device described on this forum a month or two ago. Essentially this was a white card box which stands on top of the negative on the flatbed.
It overlapped the negative all round and was triangular in the long section (about 6") and rectangular in plan and end section. Guessing, the apex of the triangle was about 110 degrees.
What it did was to give a smooth reflection to the light, but away from the negative so that no blemishes on the white card showed on the scan.