if your talking about plans ,template cutouts e,c,t its going to cost you ,have a look here it's not a 120lt board but it will give you the basics
www.seabreeze.com.au/forums/Windsurfing/Gps/Speed-Board-Build-HD-Creeper-48/, I have made a few boards and personally for what you pay for a used board I wouldn't start from scratch
there are heaps of 120lt boards out there that are all different lengths and widths , im not sure on what your intentions are but the rocker on different boards haven't changed a great deal except the boards are wider,
if you can get an older board for say $200 and do some mods , cutouts , steps , lower mast track ect, you will end up with a better board than doing your first board from scratch, and it will give you an idea on how these changes make the boards performance , then you mite want to have a go from scratch