Just had a dig though my detritus and found that lawn mower handle bolts/IMG_20171222_135527452.jpgIMG_20171222_135527452.jpgknobs have an M8 1.25 thread.
Drilled a hole though a tow ball nut wrench I got for $1 for the strap

The lawnmower knobs are probably a nice solution if yours are missing, readily available and nice and thick to grab under the seats