When it comes to weed fins, Tectonics Weed Demons are probably the best fins you can buy for single fin riding. I've got the 27,29,31,33 and 35 cm, that I use on my 95,106 and 125L boards. The specifics:
27 cm: 4.7-5.3 on a 95L
29 cm: 5.7-6.2 on a 95L-106L
31 cm: 6.5 on a 106L
33 cm: 7.5 on a 125L
35 cm 8.4 on a 125L
I think that a 29 cm would fill most of your needs on a 105L board, 27 and 31 cm would be perfect.
I use Makani fins when I want to ride 3-fins on my 75L, 86 or 95L. Their WALU Weed side fins work well. I use their AHI Weed 20 and 22 cm on my 75L and 85L, that I like to use with 9 or 10 cm side fins. I don't know what to recommend for a 105L board.