So,I've built a P3 steel V1 frame,but the open top part and no radial support for the leadscrews is a bit a bullshit. I just took a F688ZZ ball bearing and put in place to support the leadscrews. That's all,simple and functional. As always inventor make easy things in difficult things in exportation process,scale up x10 and u'll find the right dimensions.
ps I added a Version for v2.1 frame users who would like to try a ball bearing "constraint"