Flip the nut, in the fourth picture, holding the whole thing on over, it will have more surface area touching the actual cutter and will be safer that way.
I've used them several times digging out rotten wood from floor joists, they work really well and man to they remove material. For your seat bottoms go real easy at first, in fact you should practise on some scrap first.
The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
William Arthur Ward