This is a gray card created using pure RGB colors in Corel Photo-Paint. I got the settings by scanning a Delta card and then blurring and reblurring the heck out of it. Results? Each color was right around 100 on the RGB scale.
The spectral range for each color is very narrow on my card. Use it for white balance, too.
You can make a superior "card" over Delta and Kodak because it has even tonality instead of a lumpy and shiny surface. Put it on a good quality matte paper, or inkjet to fabric! If you use a 90% reflective paper, you now have the flip side of a Kodak Gray Card.
You will probably need to experiment with your printer settings of paper type and ink load. Make a small image for testing so that you don't waste ink, then when you have it right, print it at full size.
Don't forget, you just need to get the reflectivity very close, not perfect. There are many other variables of greater concern during exposure determination. I'll bet Delta cards aren't necessarily exactly 18% - but they work fine.