This might be similar to what historical is mentioning. With limited tools maybe you could make one like sketch, bolted together sandwich of for example two 1/4" plates vs. filing notches out of a single chunk of 1/2". I think this would give you a flatter contact surface for the underside of the compound Tee. The middle part is just along for the ride for general alignment & to give the post some meat to thread into. The trick is tapping the center hole through both plates. I think threading should work ok if they are securely pre-screwed together.
I'm treating this as a temporary thing. But you could also turn a T nut blank in your lathe once you have something in place & lop off the ears like picture.
You could hacksaw & file a solid but one consideration is even if you are out by a couple thou, you may only have one ear in contact with the compound & not really know it. The toolpost would probably be mounted well enough but I always consider any unequal forces like this on cast iron. Some people have cracked their mill tables with poorly made tee nuts. They don't fit so great, so they gronk them down and then ohsh*t. Cast iron doesn't like that too much & repair would be pita.