Just about to make one for my new chair. Basically I drive into place. Then my carer uses a marker pen to mark and measure where the bolt needs to be...
THEN I get a bit of 3mm steel cut, from
http://www.lasermaster.co.uk the same size as the bottom of the battery tray. Drill a 6mm hole in each corner. Weld a deep 16mm nut in the correct place near the middle to match the marker pen. And then bolt it to the bottom after spraying it black, by tipping the chair over without its batteries. And we are done! Fit tow bar bolt the correct length with loctite.
Say 260mm square? Thats £12.91 delivered plus 1 nut at £0.40 pence, and a £1.10 bolt, and a £
to get someone to tig weld the nut!