In the field, just use a log, a tree, a tree limb, your knee..there are all kinds of solid objects you can rest that head on.
That arrow grabber is great for double bevel heads.
I just do without it since I use single bevels and just run the flat side of the head across the file to remove the burr that's created by pulling the head across the file on the beveled side.
I spent a lot of years sharpening Zwickeys and Magnus 1's in the field...and my knee, or the side of my shoe tucked up under me were always available to keep the steady angle.
Glad you got it figured out. If you keep after it you will be able to get it surgical with little effort...just don't give up.
When the country breaks down as the economy fails and we all have to resort to being subsistence hunters and gatherers-being able to use a file in the field will pay big dividends.
Could mean the difference between your family eating well and having to become vegetarians.
How's that for a prediction?