I have successfully used the impression mat from global sugar art dot com. I use the Wilton crusting buttercream. I find that if you have patience (which most of us don't) it works quite well! I leave all my cakes in the fridge for 30-60 minutes before even smoothing them. I have a scraper that I've been using for the sides of the cakes to try and get the buttercream even... THIS IS KEY!!!
Then when the cake is crusted, you literally have minutes! Have your impression mat ready. I dust mine with the confectioners sugar / corn starch 50/50 blend first!
Immediately when I take the cake out, it's crusted but it's much cooler than the air outside of the fridge, so you have to work carefully. Once the cake starts to sweat, all buttercreams and fondants do when you first take them out of the fridge, you'll have to stop and rechill it. I also use a fondant smoother on the outside of the impression mat. You don't need to push hard, just gently, else you'll push the buttercream up the sides and you'll get a ridge around the top of the cake that's higher than the center.
DO NOT go to the ends with the impression mat cause not only will you leave the edge of the impression mat's line on the cake but it's much easier to line up the mat for the next section of impressions...
does this help?