I've made 2 different masks for the view screen on my c330f. Used a sheet of black foil to cut an 8x10 mask and 5x7 mask. I find it easier to compose with the aid of a crop based on my printing tastes. The square format works well enough for portraits, but I generally try to print with off the shelf frames and mats in mind.
Have 65mm and 135mm lenses and will keep this setup. It is handy when I need better resolution than 35mm, but not the bulk and weight of a 4x5 or 8x10. Best of both worlds. Good luck with your choice. tim