I've not had any problems with light leaks in my Holga WPC. I shot it on WPPD a week ago, https://photofying.wordpress.com/201...hotography-day .

However, I have not tried the 6x12 format, but use the 6x9 mask that comes with it.

I have done the HW mod that reduces/removes the vignetting, but I have not shot any 6x12 with it yet to see if it works.