Good idea! Have never made a whole handlebar but did make, or rather convert, this stem.
The main thing with fixturing is you want your handlebar to be level. Because it's quite long any small errors will be magnified.
I put the handlebar into the stem and just perched it on the table with a few magnets and things while getting a couple of tacks on. With the bar in place it's easy to see and measure the gaps right at the ends. If necessary it can be bent a small amount between tacking and welding, although I didn't need to do that.
With your design I would attach the v shaped bit first, just completely flat on the table, and then add the bit that goes over the steerer last since now you can check for level across the whole length.
https://www.bikeforums.net/framebuil...onversion.html