Let's get to the real issue; Savage Worlds don't include any rules for crafting or building, so we're inventing a house rule here. You can do whatever you like in your game, of course. My take on the issue is simply that three stars in Harn means an average skilled craftsman, and a d6 in Savage Worlds rules means an average skill...for a human. Everything else stems from there. To ensure that a house rule maintains some consistency, I always try to examine the boundary conditions; in this case the lower end is more important than the high end. If the boundary conditions present a reasonable outcome, then the house rule is internally consistent and therefore usable, otherwise more work or a rethink is needed.
Remember the guideline on skills in Savage Worlds; no chance of succeeding (ie, brain surgery or something equally complicated) means the hero doesn't get a roll at all. If there is any chance of success, but the character has never been exposed to formal training, then the unskilled roll (which is given as d4-2 ... for those unfamiliar with Savage Worlds, read this as "d4 plus a penalty for being unskilled of -2") is allowed. With crafts, much of it is common sense; experience builds professionalism and quality, but an unskilled person with a reasonable grasp of the way the world works, or some informal exposure to the skill, has a chance to craft most mundane things.
I don't interpret "unskilled" the same way you do. You equated "unskilled" to "haven't a clue", and reading between the lines, I interpret your "haven't a clue" as meaning "no chance". I may be drawing a long bow here, but I think that means you equate "unskilled" to mean "no chance" of success, and I don't think that's correct because it's not supported by the math (see below).
To me "unskilled" means the character has no formal training, but they are adept enough to perhaps be able to pull off a success...if they are lucky. That would equate to the handyman, or the craft labourer. In Hârn, one star means "Poor" quality. I figure a success rate of roughly 19% — see how I came to this conclusion below — equates to a poor craftsman; over 80% of the jobs he does are "failures". There's really not that much difference between an unskilled person and one with d4 in the skill.
You have to remember that in Savage Worlds, a success means "the activity was performed adequately; it achieved the stated aim". It wasn't a marvellous success, not outstanding, just OK. Equally, not
rolling a success doesn't always mean a complete and utter failure. For crafting, it might mean simply that the item has a flaw or two, but is still usable. So the chair might be rickety and have uneven legs, but you can still sit on it. The flour is coarsely ground and has hulls in it, but it can still be used to bake bread, which might be tough — even unpleasant — to eat, but it would feed a family.
In other words, the quality is poorer than a success would have generated, but the end product is still usable in some small measure.
And now to a bit of technical analysis:
Using my logic of equating a d6 to 3 stars:
* = d4-2 or 18.75%* (1:5.33)
** = d4 or 25% (1 in 4, or 1:4)
*** = d6 or 50% (3 in 6, or 1:2)
**** = d8 or 62.5% (5 in 8, or 1:1.6)
***** = d10 or 70% (7 in 10, or 1:1.43)
For completeness: d12 = 75% (9 in 12 or 1:1.33), d12+2 = 92% (11 in 12 or 1:1.09)
The biggest jumps are from a skill of d4 to one of d6 (a 25% jump, or double the chance of a success), and from d12 to d12+2 (a jump of 17%)
* This is primarily for those that don't know how Savage Worlds works; there are two concepts you must know. Firstly, a success is an end result of 4 or better, regardless of die type rolled, and after any bonuses and/or penalties have been applied. The second concept is that if you roll the maximum number on a die (known as "aceing" the roll; that is, a result of 4 on a d4, or 6 on a d6, etc.) then you get to roll the die again and add the results of both
rolls. So, in order to generate a success when the skill level is d4-2, the first roll must be an ace (a result of 4) so the player can roll the d4 again. The second result must be a 2 or higher, meaning the end result is at least 6. This is because the penalty of 2 is now subtracted from the result, to end up with a 4; a success.
There is a 1 in 4 chance of rolling a 4 on a d4, so a 25% chance (or .25). There is a 75% chance of rolling a 2 or higher on a d4, so .75.
.25 * .75 = .1875 or ~19%
I'm happy to be corrected by any Math majors out there, but I'm reasonable confident I have that right.