Probably the best tool you could use would be a socket extension for a 1/2" socket set upon which you grind down the male square end to fit your 7/16 (or whatever it is) differential drain/fill plugs.
I was young once - rather than "ruin" (in my eyes then) a perfectly good tool I had a piece of square scrap metal upon which I resolved to use a pair of vice-grips for leverage. Grinding it down as I described above, I found the sparks "thready" and quite brilliant in nature - I had chosen a piece of TOOL STEEL to make my dedicated tool. A positive case of technical undersight and capability overkill on my material selection.
Today I would use one of the Kobalt, Craftman or Husky socket extensions. All available from the "Big box" stores. Probably the shortest you can buy because reach is not the issue - rather cost is - and shorter is cheaper.
After a while of dealing with a Model A you learn that certain tools are "unique" to the beast, and you're wise to start developing and keeping these tools.
And, while one can be tempted to duct-tape and bailing wire a functional tool together from bits and pieces - one should always remember "The best tools are none too good" and that better IS better for a reason. And you deserve it to yourself - and to service your car in the best manner possible.
I take pleasure in my tools. In a way they are a measure of my own capability, as well as adding TO that capability. I hope you do too.
Welcome to the Model A Club. Won't you join us upstairs in the "General Discussion" Board? No question too small or answer too long upstairs.
Joe K