Fusion 360 is free software for hobbyists. It just takes sitting down and learning it. There are a lot of tutorials for it on Youtube. There are other free 3d CAD programs, but Fusion is the best for dimensional modeling. There is also Blender, but with 25+ years CAD/CAM experience under my...
Then buy one. You won't regret it at all. The Ender 3 V2 would be a good one to catch on sale, or even at full price. It is small, like 8.6" square build surface, but a great start into 3d printing. I am here and available 24/7 for anyone needing help with setting one up and getting going.