The composite cofactor of F1821 was the product of 66-digit, 73-digit, and 80-digit prime numbers:

66-digit prime factor:
926629759511319761571263749688279353784536192145962594710315627773

73-digit prime factor:
1796546201260714341447862829112068647047080961090427367838368332669415329

80-digit prime factor:
99025300830350774359853864280843700840901444912626200925223566436944589658818237

Thanks to Thomas Womack for performing the postprocessing. The factors have been reported to the Fibonacci project.