To make an i-cord swatch without working in the round, cast stitches onto a circular needle or a double-point needle, then work the round instructions to the end of your round. Push your stitches to the opposite end of the needle, pull the yarn loosely across the back of your swatch and begin working the next round instruction. You may want to have extra edge stitches to maintain accurate tension across the pattern stitches. Work extra sts as k1-b; I usually pick up the previous float and knit with the k1-b at the beginning of every-other round, alternating with doing the same at the end of the opposite rounds.
It seems like a waste of yarn, but if you did enough stitches to knit in the round it would be even more! Work from the opposite end of the ball in case you need to use the yarn. Also, this method allows you to use the same needle as you will use for your project. A different needle can change your gauge, even if it is the same size.