After a lot of searching, here is all the pertinent canon text on each Master I could find, either describing them in a scene, their room, and/or a general impression that can inspire what they are likely to have on them that could be used for identifying cosplay.
I’ll wrap it up in spoiler as it is a lot of text and some do come from spoiler storylets, though as I don’t differentiate or identify them, you’d be hard pressed to figure out the spoilers unless you’d already seen them. Do like FBG’s clever little hints and nudges in some of the descriptions.
[spoiler] Mr. Apples - Mr Apples seems delighted to see you. It presses honeyed wine on you and insists you try a variety of preserved fruits imported at tremendous expense from the Elder Continent. The fruits have wonderful healing properties, and taste extraordinary besides.
It whispers incautious gossip about the Masters in your eager ear. It does – a little unnervingly – place its hand on your knee several times. (Hand? Whatever it keeps in its glove, in any case.) Is this Masterly flirtation? Or is it determining whether you’re ripe and plump for some dreadful purpose?
Mr. Hearts - The room is steaming hot. A broad marble counter is strewn with knives and cleavers and gobbets of gristle. Carcasses hang from ceiling-hooks. The air smells of blood and peppery seasoning.
The Master is fiddling with a fiendishly intricate machine, as big as an oven. A clank. A whirr! Its glove is snatched into the mechanism. Mr Hearts’ gloveless finger vanishes into its hood, from where you hear sucking. ‘Finger’ may be an inaccurate word. Perhaps ‘talon’ would be better.
Mr. Cups - The ceilings of Mr Cups’ parlour soar high above you. The walls are lined with vast display cabinets, filled with crockery, pottery, and sculpture. Some are worth trading a spouse for. Others are tools of social suicide.
Mr. Mirrors - The room is covered with a tessellation of mirrors. A tiny hand mirror has been tucked into the gap between the frames of two full length mirrors. The walls filled, more are suspended from the ceiling on thick chains. Each mirror differs from the next, but all have one thing in common - from the edge or corner of each, a shard of glass has been removed. Mr Mirrors has stalked in behind you. Is that a saucer it’s tucking into its voluminous sleeve?
Mr. Fires - The apartments of Mr Fires blaze with light: gas, candle, scintillant stone, peculiar incandescent arc-bulbs of glass. You blink. It’s like the Surface in here. Mr Fires wears a hooded robe of red so deep that in ordinary light it would appear black – but here, each element of its hue is individually distinguishable.
Mr. Iron - In all the vast room where you meet Mr Iron, you see only his chair and desk; a single many-lensed instrument of obscure function; and ranks of watchful neddy men. He comes forward and hands you a note. He motions with a gloved hand for you to turn the note over.
Mr. Pages - Mr Pages harangues a gang of sorry-looking Special Constables amongst a desperate wreckage of books and papers…A hunched figure wrapped in a cloak the colour of parchment looms on your step. Ink stains dapple its drooping sleeves. Leaning forward, it whispers to you hoarsely. "Nocturnous felicitations, Sir. I was hoping to have a word - more accurately, a multitude of words."
Mr. Spices - Mr Spices’ den is a fragrant, colourful desert of seasonings. Your eye wanders over ridges of turmeric and basil; over dunes of lavender. Escarpments of cinnamon rise shoulder high. Scents press onto your tongue. The olfactory bombardment is exquisite; unbearable. How many kingdoms could this be worth? Mr Spices is standing, alone, at the edge of the mud-choked pond, gazing into the water. It is wearing many layers of cloaks against the cold air, and resembles an undertaker’s laundry basket with feet.
Mr. Stones - Mr Stones’ cloak shimmers, more grey than black. Perhaps it’s the tiny diamonds woven into the fabric. The tapping of its claws fills the gloomy office.
Mr. Veils - Mr Veils’ rooms are black. Black-black, closed-eyelids black, Bazaar-buried black. But not empty, or silent. Its residents shriek distantly, flutter in ghastly proximity. There are airs: there are voices.
Mr. Wines - There appears to be some sort of banquet going on in Mr Wines’ rooms. As one, the guests rise and toast you. There is a rousing cheer. Everyone’s favourite master, who probably doesn’t eat children; only occasionally has innocents beaten by his neddies; and almost never ruins small shopkeepers with savagely devious back-room deals. He keeps a fine cellar, does Mr Wines. You are encouraged to taste its products until your head swims. You enjoy a pleasant half-hour listening to Mr Wines’ tales of previous cities: the fermented mare’s milk he sold in the Fourth, the maize-wine of the Third, the sour beer of the Second. Mr Wines presses a wax-sealed bottle on you as you leave. "I think you’d look charming in black," it says enigmatically. [/spoiler]