The thing is you'll have to test for each block in the player's radius, which is huge
The best way would be to do some system which will make the player summon an entity at each furnace he places (like replacing the inventory furnaces by custom spawn eggs, custom item frames or whatever which will spawn a furnace when placed)
From this, you would be able to test for each "furnace entity" then detect them a lot more easily
If you make a texturepack, you could replace an egg's texture by furnace texture so it would look normal...
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If you know there is a contest for winning a minecraft account, I'm interrested!
Otherwise, if you are interrested to buy an account for me in exchange of command/datapack making, I'll be happy to work for you!