Junkyard Machinist

Out of all the Achievements Required for Mecha-Done to get the mount, this one will take you the longest and be most the frustrating. Completing a construction project means you have to be the person to finish the project, not just contribute to it. Some days with the smaller projects scattered around the island active will make this easier than others, however expect a lot of competition and A LOT of spare part farming.
Honestly, it does feel like this Achievement needs to have it's requirements lowered or condition changed, as it by far exceeds the investment the rest do.

Edit: As people have stated in the comments. The projects no longer require you to finish them, just contribute to them and be in proximity when it is completed to gain credit. This is a big improvement over the previous condition, however will still probably take a while.

Here was my technique:

First some information.

1 - Be a day when flame turrets are up.

2 - Note the map of where the Automated Flame Turrets can spawn, copy them into Tom Tom.

3 - The turret spawn in groups of three - there are six such groups around Mechagon.

4 - If someone of the opposite faction to you builds a turret it will be attackable and killable by you, if someone of your faction builds it, the other faction can kill it, otherwise they will despawn in 10 minutes after being built.

5 - Flame Turrets can spawn, from my experience within 1-3 minutes of despawning, regardless of how long they were active from the previous build.

6 - When a Turret dies it will pop up in one of the three possible spawn points for the area.

And how I did it.

1 - Make a macro /t Automated Flame Turret

2 - Fly to the middle of one of the groups and use the macro.

3 - If you do not find an active Flame Turret wait because one will be spawning soon.

4 - If you find a hostile Flame Turret, kill it and then wait.

5 - If you find a friendly Flame Turret you will be able to see by looking at its debuffs how much longer it will be active for, judge for yourself whether or not its worth waiting - if it was more than a minute I moved on.

6 - Travel in circles around mechagon going from group to group using this method.

I usually got one build credit every 3-5 minutes on average doing this. However there were a couple of opposite faction people also building turrets. I think this actually sped up the process, quieter or lopsided servers might be slower for this achievement.

The group in the Western Spray and the group on the western edge of the main island might be slightly too big to find a spot where you can stay still and see all three spawn points.

If you are going for the Junkyard title and are in no rush for the mount I'd probably suggest not bothering because you'll get this eventually while doing Head Financier of Mechagon

Locations of all Automated Flame Turrets (groups of 3, only 1 will be active in a group)
/way 16 81
/way 22 77
/way 20 70
/way 42 35
/way 40 39
/way 46 47
/way 55 57
/way 53 60
/way 57 63
/way 68 62
/way 71 60
/way 76 59
/way 74 45
/way 70 47
/way 68 42
/way 61 28
/way 58 25
/way 53 33