any suggestions on how i'd go about building something on linear rails?
Many different ways to go about it. At the end of the day you need to sit down and list your minimum requirements:
What size and length parts do I need?
What type of center or locating device will be engaging the part?
How quickly do I need to reposition?
Will it need to be automated/integrated with the control to prevent loading errors?
"Quill", Carriage, or both?
Do I need to limit the stroke?
If you answer these and more, you will find your design options narrowed down vastly.
I have built this very thing in the past. Actually will probably build another one soon. I hemmed and hawed over details for a while, but eventually it just settled into a given form. From there is was just selecting hardware and building around what was available. If it does exactly what you need, chances are the cost is irrelevant or negligible. Don't worry about making it super accurate as a bolt on assembly. Consider building it up as you go using the machine for measuring and qualifying certain + tolerance features or "machinable adjustments" in place. Makes it much cheaper to build, and gives you the best end result. All you have to do is keep it within your capabilities.
Everyone wants something that runs perfectly true. But at the end of the day, how good does it really need to be?
For mine I machined the "spindle" base height to control the center height and XZ plane error, but have adjustment screws along x that control the rotation about z and y axis center position. It run's pretty good. The base plate the the rails bolt to was surface ground and within the range that we cut parts in had about .0003" of center deviation, that would be combined vertical and horizontal change, as measured using an indicator on the spindle head traversing the X in multiple locations. There was a little rise of about a thou at the end of travel, but we never cut parts that small. That would also be .001" over 25" in this case which IMHO would be very acceptable in most cases, especially in this one if we ever cut a part that long.