I'm not sure there is a miniature horse stall guard. I think what matters mostly is the size of your door where you will be hanging it. The width. I've used them on my quarter horses over the years just to allow some extra air circulation in the hot months and they worked fine. On the minis, it was a joke. Those little beggers can get out of them and get a hoof stuck in them bigger than you can say "don't do that".
I have one thats solid doesn't have the holes for feet to get stuck in and it works great its one like the use for race horses and jumpers. It was awesome when my mare was on stall rest for an injury even though she can get her head over the door this allowed her to look out and see and watch the world go by. A lot of it depends on the size of the door. I've seen a lot of mini people buy gates like this http://www.mcssl.com/store/davismetalpowdercoatingllc/catalog/product/b69e35cec25240b288904e92030d497a and use those for air flow and visibility.
Thanks guys for the ideas....I was thinking along the lines of what Marty posted, though the other idea is nice too,and may be better to keep other critters from wandering in...I was mainly thinking of being able to keep the door open of my little barn at night when it is so hot...