So, after much deliberation, I've decided to go with a 2" x 6" x 3/16" x ~750mm long piece of mild steel for the X beam. I found a local machine shop willing to flatten the face where the linear rails will go and make the bottom perpendicular along with plates that I'll weld onto the ends for mounting. I found a good source on eBay for the steel. The reason I opted for 2" x 6" x 3/16" was both budgetary and design based. I couldn't source the size tube I wanted locally so shipping was prohibitively expensive after a certain wight/size point, massive jump in price. Literally from around $36 shipping to well over $100! And, since I'm doing this upgrade in increments, I'll eventually switch over to a fixed gantry with Y table movement, I want to keep the weight somewhat reasonable. This beam is 9.42lbs per foot making the total weight around 23.5lbs. That's a goodly chunk of steel to be throwing around along with the whole X/Z assemblies and router! Ooftah. Let's see what these poly carb wheels can take (on the gantry plates). The total upgrade is going to be close to $500, not including all the swearing I will do. I'm teaching myself to weld with a pitiful little DC arc welder I picked up, and off to the races! Wish me luck! Video to come.