i use threaded rod / nuts / big washers from the homemade headset press i already have. setup similar to the OPs version. i like it internally clamped since i often take the bike out of the work stand and rest the bottom bracket on the floor and use my foot / leg power to slowly turn the fixed cup.

i've overhauled a few dozen 30 year old bikes and been able to get all of the fixed cups out so far. liquid wrench 24 hours before removal often helps.

a good tip is to tighten the nuts slightly past finger tight and then the fixed cup tool won't slip and mar the frame. nuts can be easily snugged and removed with a big adjustable wrench.