I've been thinking about your question for a while and just off the top of my head here's what I would try to do to make the
back of the gauntlet opening larger.Our gauntlets were MOW so very sturdy and easy to work on.
First fit the top and bottom halves as you normally would. When the seams are straight and perfect the rest of the work is
just on the bottom half.
Get some sheet styrene the same or a little thicker than the wall of the gauntlet and pattern and cut a wedge (triangle) the
length of the gauntlet seam,narrow or pointed at the wrist and expanding the triangle as needed to be taller at the rear edge of the gauntlet.
You do two exactly the same, one for inside and one for the outside bottom half seams.
Epoxy them along each seam and reinforce them with fiberglass cloth or tape and resin on the inside. I've done this sort of thing
with a small wedge like this it will be strong.
Sand this new edge to fit the top half seams. On the outside fill and sand as necessary to loose the repair.
This will expand the vertical size of the rear opening to whatever you need. Can also enlarge the wrist opening as well,if you want to.
Most big box or auto stores will have small fiberglass kits. Hobby stores or on line for thick styrene sheets.
You probably know this ....to get good matched seams tape sandpaper on a very flat surface and move the gauntlet halves over it
test fitting as you go.
This is just one way to go. Hope it helps you moving forward.