Stern rail seats

Has anyone got these?
We have a First 325 1987 and being short (barely 5ft) I find it difficult to find a comfortable spot to perch when cruising. Resulting in a sore backside after a few hours.
Does anyone have any experience of these, where to buy and how best to fit? Or are they just too much of a hinderance?
Any advice or opinions welcome. Thank you.