Have a look in Fairways in Coleford or if you happen to be in Cheltenham pop in and see Steve at DCM, you could park outside and they might let you try the product for size.
Our need was temporary following a hip replacement. We were advised to check the local Red Cross's equipment loan volunteers what they had. I took Thetford's seat down there and found a hollow blown plastic "topper" that was ideal. I did make it extra secure with a rubber bungee tentioner at the rear. It was rigid enough to be left for both of us to use over the period it was needed.
We were loaned this for four months for which we made a donation.
It had the makers label and fitting instructions on the bag, so if needed just to track down what fits then the Red Cross may be helpful.
Wonder if you can let me have the makers name please, if I can't find a riser I may not be able to go on holiday in caravan for a long time.
Many thanks,
Sandra