DIY Power cables..Are metal end plugs better than plastic?

If SonarQuest are built like Wattgate, you can simply knock the plastic strain relief piece out to accommodate your larger cable, it will then not look off centered. I wouldn't worry too much about the strain relief unless you plan on unplugging by grabbing the cord.
As for the power connector casing, I prefer a case made of non-conductive material.

Some smaller companies like Cullen regularly use SonarQuest on their products.

I made a few power cords with SonarQuest plugs and they were easy to wire and have been holding up with no problems.   I assume your cable is held tight so having the strain relief off centered is just cosmetic.  If it bothers you just put a layer or two of heat shrink tubing around the wire to increase the diameter.  But no matter what plug you use or how tight it is never pull on the cable to unplug it.   Always grab the plug (on both sides).
I think the SonarQuest plugs and they are the best bang for your buck out there.  The plastic is not in the same class as more expensive plugs but the internals are heavy duty and thick and it makes a firm connection with the hospital grade outlet I have..   I use the pure red copper plugs and for $20 a pair it is a no brainer to try.    "Don't cost a fortune" is a relative term but I didn't see any comparable plugs with copper internals that cost 3-4x the cost.    And most are much more than that.

The SonarQuest plug cases look a lot like the Wattgate cases.  I wouldn't worry too much about this.  They are durable enough to hold onto as you plug/unplug your power cords.  I don't really use the strain relief on power cord connectors because the internal wire clamps are usually very strong.  Strain relief becomes much more important on more fragile wiring such as interconnects which are only soldered onto contacts and the extending cable needs to be held "straight" so that it doesn't stress the solder contacts.