You can make a negative carrier out of mat board but you don't need to. You can just put the negtive directly on the scanner glass. You should either tpe it down using blue painter's tape or wet mount it using a light wet mounting spray such as Prazio's Anti Newton Ring spray.
There is a lot of discussion about fine adjustments in film height from the glassd to get the best focus out of an Epson 4990 but IMO it isn't going to matter very much with a 5x7. I have frequently taped 5x7 trannies to the glass of a Microtek 1800f, same general principle but a different scanning path.
BTW, what is the ultimae purpose of these scans? That has something to do with how you will scan.