There are some great images and tips here. I too, built an aluminum extrusion CNC based on the Kyo Sphinx design. It is a good machine and has worked flawlessly for most woods, acrylic, etc. But I'm starting to mill metal and, well, there is where the c-beam aluminum extrusions start to break down. There is chatter, x-axis flex, etc. I'm currently looking at ways to strengthen the machine without breaking the bank. I'm going to do a video on it soon and post some prices, etc. You can check out my trials and tribulations along the path and please do give suggestions if you have them. I'm thinking about using 2" x 4" 11 guage rectangular tube for the X but I'm worried that it won't be level or square, but then I can just file it and shim, etc. I was thinking about going with v-track and v steel bearing wheels as I've done on the x-axis, but I have to do a price comparison with Hiwin type linear bearings. I'm surprised you don't mention v-track as a linear solution. If you care to follow along on my journey, take a look at my youtube channel: https://youtu.be/2QTZa2Ge9A0