My horse has a fairly big mash, about 3.5 pounds dry weight (alfalfa cubes, beet pulp, oats, supplements).
I had a plastic corner mounted manger that worked fine. I took it down last summer when I was power-washing the stall, and didn’t put it back up. Instead, I went to a flat black rubber pan. Horse is too busy eating to play with it until it is empty and licked clean! The pan fits her current mash, but would be too small for 6 lbs of cubes, I think. But you can get a double-height pan.
I don’t find any feed gets wasted. But the part of her stall where she eats is swept clear of shavings before I feed her, and then I usually try to plop the pan on top of her hay. So if she dribbles into the hay, she just eats it up, and she sometimes licks the rubber mats clean. She has a run-out paddock and isn’t a stall pooper, so her eating area stays clean.
I went with the rubber pan for two reasons. First, I can scrub it out after every use, while the manger was a PITA to get off the wall. She licked the manger clean inside, but the outside got a bit slimy. Also, maresy hated the mare next door staring at her while she ate (there is see-through wire on the top half of the walls between the stalls) and tended to toss her head around, snap her teeth, and ended up spitting feed everywhere, all over the walls (then the neighbor horse licked the spit-out mash off the wire, and that made maresy even angrier). When she eats off the ground, she doesn’t see her neighbor and is much calmer.
However, when feeding other people’s horses, I don’t like the rubber pans so much if you have a horse that poops all over the stall, and especially if you have a horse that poops in the pan! Very gross to deal with when you’re just trying to do a friend a fast favor, way above the call of duty IMHO 
And there is something about those rubber pans … they can be very hard to get totally clean, for some reason. You can be scrubbing and scrubbing and the rinse water is still greyish. They pick up all the dust from the bedding and even the scum from powdered supplements, etc.