This style in general, flipped upside-down for hoses on driver's side, was what I was thinking. I'll have to double check on the 12" guesstimation spec...
310mm x 524mm (12.2" tall x 20.6" wide would be good for a TDI)
https://www.ebay.com/itm/2-ROW-ALUMINUM-RADIATOR-VW-GOLF-GTI-JETTA-MK2-Corrado-Scirocco-Coupe-1-8V-86-95/251303341144
https://www.ebay.com/itm/Aluminum-Radiator-for-VW-GOLF-GTI-JETTA-MK2-Corrado-Scirocco-Coupe-1-8V-86-95/201485961017
or 28"W x 12.5" tall if this could fit height-wise over the steering gearbox and under the hood, this would provide the most cooling and have the hoses in a preferential destination:
https://www.ebay.com/itm/Aluminum-Radiator-for-VW-Jetta-Golf-Mk3-1993-1999-1-8L-2-0L-4cyl-MT-1995-96/222646634103
This may be the best all aluminum option if 12.5" will clear between the steering gearbox and the hood bracing... or trimming the hood bracing even.
There was still yet another VW/Audi application that I saw that I recall seeming far better in size, but I only could find $300 versions of them... and this was before I measured under my hood to get the dimensions I was targeting, so perhaps it would not be such a good fit, but perhaps it would...
There are Jeep Cherokee radiators 36x12 but that would nearly span from the center of one headlight to the other...
Here are some of the WIlly's and Willy's Jeep options I saw:
This one was darn near textbook perfect except for the upper hose connection being dead center. The bottom cutout notch even corresponds nearby the Samurai steering stabilizer mount! Some TIG work to relocate the upper hose connection? available hoses to figure out the maze/puzzle will have a lot to do with 20" wide vs 26" or so wide options. Acme has it figured out with the hoses for the Quantum radiator they recommend and sell.
https://www.ebay.com/itm/3-Row-Radiator-For-51-53-Willys/181779316780
or
https://www.ebay.com/itm/3-Row-Western-Champion-Radiator-W-12-Fan-for-1951-1952-1953-Willys/202569353623
Running one like this could be great as it would leave some extra room to the driver's side grill opening above the steering gearbox to fit an intercooler with direct airflow to it, but it is on the wrong side of the engine for the intake piping, so things would get cluttered if trying to plumb that up. This core size is going to give the largest cooling area possible for considering being exposed to the direct airflow through the grill opening at least.
Or possibly some TIG welding work on one of this style of Willy's Jeep aftermarket aluminum radiator, the hose connections are in the best place they could be for this width of radiator, but the funky bends on the stubs is odd and may need chopped/TIG'd? If plumbing to this location is far enough to the driver's side with available hoses... I think it is, but the diesel injection pump hangs off the left side of the engine and may complicate things???
https://www.ebay.com/itm/Aluminum-Radiator-For-1941-1952-Jeep-Willys-M38-CJ-2A-MB-42-43-44-45-46-47-50/322792012559
(Lower hose 1-3/4" though)