I’vs just downloaded the free version of Shape3d Lite and am using it to visualize the modifications I plan to make to an old windsurfer before I start cutting. So far I have found the program super easy to use and very helpful. Only thing missing – best I can tell – is the ability to incorporate the cut-outs for my waterproof boxes into the design.
This program also allows you to design a board from scratch and send the design to a local cutter with the proper machine. However others on this site have said those machines may be limited in board thickness. The board I am modifying is 5.5" thick.
Good software, I designed and realized a few surfboards with that software, a lucky one is still my favorite 6’6" very small wave! To be able to draw cuts like you need, the commercial version is needed. Freeware can’t do, no matter for how many slices you’ll add it will interpolate them, leading to a mess.
Check with the shaping factory if they accept such draw… Front and back recess wall doesn’t look like good, it’s a 3d software if something looks weird…usually it doesn’t print/mill fine. You need the commercial license of the software.