maby try and use your old tube or find another tube and just cut it off about 1"1/2 from the block and pinch it shut and maby weld the end so it for sure wont keak
a steel tapered plug would tap(i mean with a hammer, not threaded) in that hole nicely...that all depends how much you care about the hole where your dipstick was..which probably cant be seen anyway, hope you're replacing the rear main just because, mine started leaking..