I have a little bit of experience with this. My dummy is one that was imported from China and made from old temple pillars, so they were fully cured and it's really nice, but it cost me about $1,000 and we coordinated an order for multiple people and even multiple area schools and waited a good while, so shipping was reasonable.
Another local sifu's students bought him a really nice one in a garage sale.
So it is possible to get lucky.
But, that's not what you asked about. I've seen a few PVC ones and they don't come close, but for practicing the form, they're useful. You're not going to be able to wail on them and expect them to hold up, nor will they give you the right feedback, but footwork, range, repetition of the form, it's not a waste of your money as long as your expectations are reasonable. I don't think the material is all that important, cutting the angles right for the arms will be.
I've seen advertised some "arms" that clip onto bags or a post and I think they would accomplish the same thing. I have a utility pole in my front yard and I've thought about mocking up some kind of arms and leg that I could affix to it so I could go out and run the form a few times and take down. I think it would work, but it would not enable me to train the way that I was taught to on a mook jong.
One last example, regarding building a wooden one. The trunk is the problem. Arms and the leg are easier if you you have a lathe or they can be bought. The trunk is supposed to be hardwood and either dried in a very large kiln or aged for decades, so that it doesn't change size and shape. Cutting the slots at exactly the correct angles is very difficult and I'm not totally sure how it's done, especially in old hardwood. An student of my SiFu was a extremely talented cabinet maker, very in-demand and had a professional shop. He made one, but he used a softer and greener wood for the body and he wasn't able to figure out how to cut the slots for the arms correctly. They should be at very specific angles, though that does differ a bit by lineage, and larger than the appendages, so that they move in the slots as you express force on them. He didn't get the angles right and it sucks to do the form it as a result, you contort yourself into odd positions, which is essentially bad training. Also, because the wood was too soft, the shape of those slots has been rounding off over the years and it's getting sloppy. No one likes doing the form that dummy. Personally, if that's the best he could do, I think it's a long shot for the average weekend woodworker to do it passably well. Probably if he made a few dozen, assuming he could get the right wood and season it appropriately, he could do as well as anyone, but, getting it passably right on the right first was not how it played out for him.
If you are a Wing Chun student has been taught the form and how to train on a dummy and this device will be your only mook jong, I don't think you're going to be happy unless you buy a proper one. If you're looking for something to practice the form on at home and you have access to a good jong at your club, it might be okay. If you're not a Wing Chun student who is being taught this form, but want a dummy to work out somehow, I'd look at one of the wall mount ones or bag mounted ones honestly. You don't have to say, I'm not trying to out you, but your profile was empty, so I just didn't have context. Feel free to PM me if you'd like to discuss and don't want to disclose publically. No judgement from me.
Otherwise, hopefully something in there helped.
