I've emailed Stag Arms for help and got zero response.  There's nothing at all on google or youtube.  Because Stag doesn't sell a vise block to fit their left-hand uppers, I had to rig one from a Midway USA block.  THe only thing was, even after cutting away a bunch of material to make the block almost work, in the end I had to remove the dustcover from the upper in order to use the block.  The Stag left-hand dustcovers attach at the top and open upwards, opposite of a right-handed upper.  Right-hand instructions for assembly don't really work.  I've spent hours trying to attach 2 dust covers.  I somehow got one of them on and working a week or two back but the second I just cannot get.  I have 5 more lowers I am going to build but probably not from Stag if I can't make this work.  Any suggestions?  Anyone have experience with this?

Cheap and I recommend: https://www.plastixrevolution.net/products/upper-receiver-vise-block

Cheaper without "not needed" torque plates. Got mine on ebay. Very handy for lefties. https://www.youtube.com/watch?v=-aqrZI0ylN4

Related story: I have several lefty uppers. Mostly Stag. Due to the configuration of their dust cover and pin, when used with most aftermarket handguards, the pin will walk forward enough to partially drop the cover or it'll unass the cover completely. If used with a basic handguard, the pin will stay retained due to the delta assembly. I remedied that by going to the Strike Industries AR Enhanced Ultimate Dust Cover. https://www.youtube.com/watch?v=BD35B6RKKRo I use their polymer versions. Great design. The design should be used by all. In order to use it in the "flip-up" mode, I had to trim a little plastic from the left/right sides of center. Very simple mod and the cover fits perfectly.

I use and recommend barrel clamping jaws in a vice.
You have to install the dust cover before the barrel anyway.

I ordered the plastixrevolution combo AR-10/AR-15 pin vise block.  Too late for the builds I did today but it will be handy to try on my next ones.

I have my own related story to plastixrevolution.   I ordered their hammer block from Amazon.  it wouldn't fit into the space between the upper pin holes.  I double-checked the reviews and it appeared this issue had been going on for 3 months.  I wrote a 1-star seller review which seemed to finally get their attention - I guess they weren't paying attention to the 1-star product reviews they were getting but probably get a notification from Amazon on 1-star seller reviews.  They contacted me and offered to send a replacement, saying that they had a defective order of stock from which they cut the blocks.  Well, it was quite clear that they had far more problems than stock - they had a defective testing and QA process as well to be putting out a product that didn't work for that long.  So I told them that if they sent the replacement and it worked, I'd update my product and seller review.  They sent it and it worked.  You can't change seller reviews, it appears, on Amazon so I could only delete it and then I upgraded the product review.  It was good of them to fix the issue promptly once they identified it.  Hopefully, in the future, they'll do a better job of testing their products and also of watching their product reviews.

Their quick response, once they finally woke up to the problem, was excellent and I am definitely willing to give them a second shot - and just did with my order for the block you suggested.

Along with the plastixrevolution block, I am also ordering this.  The obvious advantage is that it allows me to tighten the flash hider without damaging the barrel extension.  Far cheaper than a damaged barrel and/or lower.  Thanks.

I got both clamps in.  I didn't use the plastixrevolution one yet but I did use the barrel block.  It is totally unsuitable for my 11.5 inch barrel because I had to take off the gas tube to get a flat barrel section long enough to hold the block.  Between the gas block and the flash hider is too short on my AR Stoner M4 profile 11.5 inch barrel.  For standard carbine or longer barrels, it probably would work better.

I literally pulled my weight off the floor on the vise handle trying to tighten enough to get the barrel block to hold and it still slipped trying to put in the flash hider.  I removed the hider and tried again with a crush washer from a different supplier and it tightened fine so I'll blame that on the washer not the block.  Problem is, you'll ruin the finish on a barrel before you know the crush washer is too stiff.

But, in the end, I was able to get the flash hider on both uppers.  One I had to remove the gas block and tube.  The other I hadn't installed them yet so it was easier.

I'm starting some additional builds tonight so I'll get to test the plastixrevolution block for barrel nut tightening soon enough.  I used a similar block on my first AR build  in 2001.

Thanks, all, for the suggestions.