Been there, done that, doesn't work. I had the exact same thing you are describing welded up by a pro. It lasted less than two minutes. Took it back to him and he welded the dickens out of it, that time it lasted maybe five minutes. I welded it up a third time inside and outside the cup using 6011 rod with 100 amps and again it broke. I was also using a HF electrioc hammer.
You best bet is to either by the cup, $60 to $70, or rent one. I did that for $12 for the weekend.
One last thing. Trying to lift, hold and balance that 80 electric hammer on top of a wobbling t-post will beat the you-know-what out of you real fast.
QRTRHRS, your best bet is to rent a pneumatic rotary hammer drill with a 24 inch bit, drill a hole, then drive the t-post into the hole the old fashioned way.