Jacks are deceivingly tricky. They seem really straightforward but alignment, action, blade profile and blade composition are all pretty exacting if you want them to work well. I've made a ton of tools and have a great time doing so, but there are some tools I'd rather buy from expert makers and most bench hand tools fall into that category.
If you'd rather make your own and find the challenge fun, great, but if you appreciate that someone else has sorted this out and would rather spend your time blowing glass, consider buying some nice handmade tools.
I have two of my favorite medium jacks for sale if you're interested. Brand new Maruko (handmade in Japan); stable and smooth on the glass. They are $430 retail, I'll sell these for $390. No doubt you could make jacks for way less, but I really doubt they would be nearly as sweet as these.