Actually I thought I checked the impossibility of that, but probably I forgot to set the faster screen fade off when I did that.
However, thanks for mentioning it.
Since I also found another backroute digging down and turning around at the bottom using mining, bashing and building, I see no other way to close this backdoor than making the upper platform completely steel (the right two blocks).
(If you have another idea, please tell me.)
Actually I didn't want to do this to make it look that it's possible to get down there. At least you could get down at the left, but it's quite obvious than you can't get further this way.
Anyway, here's the updated (and probably last) version:
http://de.geocities.com/geoo89/g0s08-3.zip