Unless you want vignetting, you want to make sure that the focal length projects a big enough image to fill up your entire 8x10 negative. Longer the focal length = larger the image. I made an 8x10 pinhole camera a couple months ago and just tape the paper to the back. This was because I don't have the skills to make something that could take a film holder - haha. I third or fourth what everyone else says about the yellow filter, seems to work well for me. Just be sure to add a stop or so of exposure time to compensate. Also move the yellow filter around during the exposure so your image doesn't have dust specs on it that are on the filter. I use aluminum from soda cans to make my pinholes. Cut the can up then use an orbital sander to shave it down even more thin. Poke a small hole through with a sewing needle, sand down the burr on the other side. Not to get into the topic of scanning but if you want to be real accurate, you can scan and measure the diameter with a photo program. Also, one of the things that helped me out the most was an exposure formula that another user on here gave me. I'd give credit if I could remember his user name. The formula is:

Tc = Tm (Fc/16)^2

Where Tc = Correct exposure time, Tm = Exposure time metered at f/16, and Fc = Focal ratio (equivalent to the aperture number of your pinhole). And then of course you need to add time on top of that to compensate for papers slow ISO. I attached a picture of my 8x10 pinhole camera and a recent photo I took with it.