I'd like to do a box-shaped cake with very sharp, precise corners. I'd seen someone else on this site used gum paste for the sides...
Would fondant with some tylose mixed in work just as well? I imagine I'd have to mix in the powder, then roll very thin, and cut? I'd adhere to cake with royal icing.
Thoughts? Suggestions? Thanks in advance

I think fondant would be fine. Just give it some time to dry so that you can keep it straight and you end up with nice edges. Good Luck!

I used modeling chocolate panels on a shoe box I did a few weeks ago. I tried it with fondant a few months ago and it seemed to droop a little. The modelling choc stayed nice and straight and neat.

I measured the height and width of my sides and cut panels approx 1/8" bigger, popped the panels in the freezer for 10 mins to firm up and popped them on the cake and used a razor blade to trim to size. (I figured a slightly big bigger is easier to trim down than being slightly off and a wee bit too small)
Good Luck

Great! Thank u!