So. a 3D printed cap gripper is my answer. Printed in robust PETG and in a bright colour for finding it easily. Tested by my better half who has never ever been able to get the water cap off.
Cost is £9.50. + P&P £1.12. Large Letter.
Comes flat pack. The two halves just need screwing together with the provided screws.
If you have severe arthritis, let me know and I can send it assembled but postage is £3.49 Small Parcel