This is a list I've been playing recently. It has strong matchups against other Shop decks (depending on how Revokers appear), Storm, and Dredge. Against the rest of the field it has opportunities to either go broken with the Two-Card Monte combs or to play a more usual Workshop win with Lodestone Golem, Thorn of Amethyst, and other attackers. Kuldotha Forgemaster helps tie things together and provides another quick-win combo with Blightsteel Colossus.
Two-Card Muddy
4 Grindstone
4 Painter
4 Helm
4 Leyline
4 Serum Powder
4 Lodestone Golem
4 Thorn of Amethyst
4 Kuldotha Forgemaster
1 Blightsteel Colossus
1 Chalice of the Void
1 Trinisphere
1 Memory Jar
4 Mishra's Workshop
4 Ancient Tomb
4 Cavern of Souls
2 Urborg, Tomb of Yawgmoth
5 Mox
1 Mox Opal
1 Sol Ring
1 Mana Vault
1 Mana Crypt
1 Black Lotus
Sideboard
4 Leyline of Sanctity
3 Phyrexian Revoker
2 Razormane Masticore
1 Platinum Angel
1 Wurmcoil Engine
1 Witchbane Orb
2 Pithing Needle
1 Crucible of Worlds
Postboard, Leyline of Sanctity (with the maindeck Serum Powder) makes the Storm and Oath matchups very winable. Revoker and Pithing Needle come in to preempt Dack Fayden, as well as to support a stronger Workshop Prison plan if necessary. Razormane Masticore and the other creatures help against Workshops and creature decks that might run Revoker or Null Rod.
Regarding a few cards specifically:
Serum Powder has been great. I know other Workshop decks have used it specifically to solidify the manabase, and it does a good job of that here. Obviously it also helps assemble one or both of the two-card combinations, particularly getting turn-0 Leyline of the Void into play. The free, personal draw-7 thing is pretty good. If you don't like the card, I would consider cutting it for 2–3 black tutors and additional Urborgs or other black lands. You could also just add more mana and cards to get with Forgemaster (namely Sundering Titan).
Tutors were inconsistent and incongruent with the Thorns and Lodestones. Obviously this is one of the drawbacks of playing this version of Two-Card Monte over the five-color version.
Cavern of Souls is really good. I've been playing them as four-ofs in all of my recent Shops builds and have loved them. Getting a Lodestone Golem or especially Forgemaster to stick is a great feeling. There's so little drawback even if they're not always able to make something uncounterable.
Memory Jar is one of my more frequent Forgemaster targets and is great even hardcast. A group of us was talking today, wondering why more Shops decks don't play it as a one-of. I understand why not, but it's very good in more combo-centric versions.
