I don't want to be a total knob but you don't sound like you have attempted anything like this and wildly underestimate the work involved. As such i think it's not a great idea to be recommending it in such a casual manner.
Despite saying in such a throw away manner
Yup, about 90 for loose, moist soil.
I don't believe that you had even calculated or consider the tonnage required.
That's why i made the insane comment. You can send me a large slice of humble pie if i am wrong and sorry to be confrontational.
I was only recommending a mini-digger if they had re-profiling work they could do in order to generate some on site spoil
I've never shifted that much spoil by hand, no! I don't mean to sound like it's the easiest thing in the world, it's not, it's a bloody stupid idea, but if you want it filled and have no access, how else do you do it, other than have a conveyor system as described above? My 90 tonne figure is based on 1.4x cubic capacity, which is a good average for loose, moist soil. If you want to compact it all into the hole it'll be more obviously.
At least this has highlighted why capping it off is the smart way to go!