I think there are two main things that cause a Fett visor (and entire helmet, actually) to look more inaccurate than it really is: camera angles and head tilt. The EFX helmet in the post I'm quoting is not being framed by a straight-ahead camera angle. I'll try to demonstrate what I mean. Here's an image I put together of the movie helmet from ESB so that I can have a solid reference for when I start customizing my own ESB Boba helmet. The yellow lines are there to line up the vertical part of the T-visor, the horizontal part, and the top & bottom of the entire helmet.
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Now here's the EFX helmet from the post quoted above (slightly tilted in order to line up more accurately) using the same reference lines. You can see that the "squint" of the visor looks too narrow because of the camera angle. If you compare it to the movie shots, you'd think it's completely wrong.
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Now here's the Hot Toys ESB helmet (both straight-ahead and profile) using the same reference lines. The squint isn't as inaccurate as when you compare it to the EFX helmet. The camera angle is still too high on the HT shot, thus keeping it from looking even closer to the movie helmet that's next to it. The actual problem with the HT helmet is that the ridge above the visor should be thicker, so as to drop a bit further down. This would make the "squint" perfect.
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And, finally, here's the same pic, but without the grid lines. I'm still waiting for a straight-ahead camera angle of the HT helmet. Every promo image is from too high, or has the head tilted down too much.
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There is a very significant structural problem with the HT helmets (including ROTJ), but IMO, it's not the "squint" of the visor (or even the ridge that is too thin). I won't get into that all that here, though, because this post is probably already confusing enough.
but looking at the HT helmet and TESB pic. The top of the T visor is straight, the HT has a slight curve.
