Questions about aligining, squaring, paralleling, self-leveling epoxy

[Dman65]

I finished building my first router last weekend which is a small Solsylva 10x9 design. I am hoping to use it to cut some parts for a new larger router that I intend to use to cut parts from MDF and hardwoods for cabinetry and built-ins.

I am wanting to make this router either 4x4 or 4x8, but 4x8 would be a tight squeeze in the small shop area I rent unless I could mount the machine semi-vertical like the panel saws at the hardware stores.

My current question comes from what I know will be the most difficult part for me and that is getting everything square and the rails parallel. I am currently looking at building using rectangular steel tubes using bolts and epoxy to join them. I have never welded before and I imagine if I start trying to learn how for this project I will never get it finished. I have read in a few places "self-leveling epoxy" being mentioned and I am wondering how that would work for getting the X axis rails parallel. The only thing I can think of is after building the rails I would need to set them on a table of some sort that has been leveled. Obviously you could just put duct tape around the edges of the two distinct rails supports and get flat surfaces, but they would no be co-planar. Would I need to run a piece of channel between the two supports as shown in blue on the third picture so the epoxy would flow from one side to the other and then they would be both flat and coplanar?

I am also wondering if there are any hints on getting the various pieces of the machine square and parallel. I have read a few things about using guitar string, but I haven't seen any images of the process and from the word descriptions I am not quite getting how it works. I was wondering if it would be possible to machine cross hairs into mounts that face one another so that on one side the cross hairs go all the way through and on the other side they are just lightly etched so a cross hair laser could be shown through the one and then aligned with the etched side of the other.

[PaulRowntree]

I tried welding to see if I could learn to do it well enough in a reasonable time. Within an hour I was getting about 50% of my joints looking good (not to say professional .. that takes years), 25% were not good enough, and 25% were not fit to be seen in public. I am guessing that another few hours of practice would be all that I need to get it to 80-90% success. It depends if you have access to a machine and the time to learn.

My steel supports for the X runs are straight enough to be used with slight shimming, so I will be building two side frames with adjustable feet and some cross-braces; they will be connected together at home and the bed installed there. So the two sides can be made independently level wrt gravity, then coupled using a sensitive level to make the two sides parallel. Finer precision comes with shims and a tram.