Voron has open source printer plans i think. The og is reprap

I've helped build a couple 350mm Voron v2 for the local library makerspace.

Those are some big linear rails!! Can't wait to see what you make with that. Maybe a belt printer would be good, lot of the huge ones are belt based for some reason. Like how about this one?

https://3ddistributed.com/mrrf-2019/white-knight-3d-printer/

I built one, but it was a (top-down) resin printer, so I don't think I have any useful advice to offer. Also, I kinda burnt out on the project after I came in one morning and found the resin had melted the vat and leaked onto the floor--somehow I just totally failed to consider that the solvents in the resin could obviously dissolve a lot of plastics. Huge mess, huge pain to remediate. At least it was a concrete floor. I got a replacement vat in glass, but I never worked up the will to work on it again. It did basically print before that, but I had a lot of off-layer curing and didn't get to do much work on tuning that in before the accident.

What size rails? Double carriages makes me think DIY desktop cnc.

I built a Prusa Mendel from scratch eons ago. With enough of effort, and a more modern nozzle and extruder, you should be good.

I have a printer that started life as an Ender 5 Plus. the print volume is 350x350x400. I have printed some pretty big things, like a full size Mandalorian helmet in one piece, and never maxed it out. A lot of my prints would fit in a Bambu lab A1 mini. Ask yourself how many large prints you'd do in one piece. Also, printing large prints can be stressful, as failures mid print are a lot more painful in large prints. You won't see many printers larger than 350 for a reason.

My printer has been converted to an Endorphin Hybrid CoreXY so of the original printer, little more than the frame, motors, and bed remains. It's a pretty competent printer. Over 3x the stock speed, good quality prints, and cost me around 150€, including an overkill (not really, has room for a toolchanger mod I'm planning) Octopus Max board. if you use a simpler board, you could probably shave around 40€. I had the printer, so it was a no brainer for me.

I got a couple of rails that were larger, because for some reason they were cheaper than the original. I cut them to size with an angle grinder on a miter adaptor which I had, but which are cheap. Just make sure you wrap the rails in masking tape, and grind down the edges a little. Any speck of iron dust will wreck the carriages.

Ender 5 Pluses are cheap used.

If I were starting from zero, maybe I would have looked into an Elegoo Centauri carbon, 310€, or an Anycubic Kobra X,339€ including 4 color MMU, or ideally a Snapmaker U1 toolchanger, 850€.

Just because I have some rails wouldn' really justify building a printer from scratch, but that's just me. To me is like justifying building a mountain bike from scratch because I got some wheels free.