Just do 2 layers of icing. You can buy ready rolled icing, it is fab and easy to use, and has the same consistency as marzipan. I'm sure you could use it without marzipan as long as you use 2 layers.
I use 2 layers of that, cut the 2nd layer smaller and cut it wavy and lay it on top of the first. Then you can use writing icing to make icicles hanging round the edge. Grate some of the leftover and sprinkle on top with some silver balls. Plonk santa etc (bought from waitrose) on top and....voila! Takes about an hour. If I was techno -able I would at this point insert a photo!
You could use one layer of the ready roll stuff and then do your frosty icing on top I'm sure.
It was difficult to get the ready rolled stuff this year, always used to get it in Safeway but b**y Morrisins discontinued it. Got it in Asda in the end. Got the wriring icing in Sainsburys.