Rollpie Cardboard is a free texture generator that runs in your browser with no install. The paper fiber, the flecks, and the flutes are all computed pixel by pixel, so the pattern never blurs at any export size.
In the design panel you pick a part on the left and set its values on the right. Liner holds the light and dark paper colors and the mottling, with the fiber and the recycled flecks one level down. Flute holds the pitch, angle, depth, and wave shape that decide how strongly the corrugation shows through the liner.
Cut edge takes a side and shows the board edge on: two liners with the corrugated medium waving between them. The spacing of the waves follows the pitch, so a narrow band with a large pitch gives a calm wave. Crease draws grooves across the sheet and bends it: mountain or valley decides which side catches the light, the bend decides how far the two faces part, and the crush decides how far the flutes fade at the fold. Stain scatters dirt over it.
In the file panel, Export lets you choose the file type and the scale and download the image. The preview on screen is not scaled up: the image is drawn again at the size you asked for, so even large exports stay sharp. It is ready to use as a background tile or as a texture on a 3D model.