Well a CNC router`s structure depends on what material you wanna cut with it, in your case it`s only wood and plastic, a C-channel steel would be a cost effective way to make it, coupled with 2 inch sized angle bars for the bed, steel is an easy to weld material and sturdy too or if you wanna make it more expensive use the stainless steel version of the material though I never built one before but I`ved built 2 units of 20 foot boat trailers, one using galvanized material and the other one pure stainless. so yes I think it`s sturdy enough to support a 5 pound moving router on my opinion.
A good example of a CNC kit it`s almost plug and play without breaking a bank I guess then it`s up to you on how you design your table with it, it can even just be bolted on a wooden table to get started cutting wood and plastic projects >>> LT4800G LT GANTRY | Pro Cut CNC