Our client has a VW T5 Transporter based camper van which had been been adapted in the past for hand control.
A combined hand throttle and brake is operated by the right hand, and the client steers mainly with the left hand, using a handle attached to the steering wheel - similar to commercially available steering wheel knobs which are popular with van and HGV drivers to apply steering lock quickly.
The problem was the vehicle horn was not included in any of the adaptations, and our client was finding it very difficult to press the airbag/horn assembly due to its strong springs - in addition to the obvious problem of needing both hands to properly control the vehicle.
The hand throttle/brake setup included some additional switches already, for indicators and headlight dip, but there was no suitable position for us to add a horn switch on that side.
The logical and intuitive position for the horn was on the end of the handle, so we fabricated an entirely new handle assembly based around the clamp from a commercially available and good quality "quick release" steering wheel knob.
The new handle was machined out of two sections of solid aluminium bar. The inner core is fixed in position into the clamp and drilled out to carry the wiring and push-to-make switch. The outer sleeve is covered with adhesive lined heat shrink sleeve for good grip, and rotates around the core, being secured in place with a circlip and thrust washer.
On most modern vehicles, the horn is not driven directly from the horn plate. Either a relay or (in this case) an electronic control unit handles the higher current to the horn, so it was possible to run light duty wiring from our handle/switch up the side of the air bag assembly and connect in parallel across the terminals of the horn plate. This retains full operation of the horn via the plate for MOT purposes. The wiring was further protected with heat-shrink sleeve.
NOTE - this project required removal of the vehicle airbag which contains an explosive charge, and should not be attempted without reference to the vehicle manufacturer's instructions.