On our trip home from the Mothership, Liz had a problem with the height of the seat bothering her knees. The short-term solution to get us cross-country was a cardboard box with some wood in it.
When we got home, I built this simple little footrest. It's about 3" high and Liz no longer has leg problems. It has worked well in both our Front Lounge and now our Rear Bath.
Here's a view from inside showing how it is curved to fit around the doghouse. It's essentially the same shape as the LD passenger-side footmat, but a little wider on the door side.
This shows how the front edge is cut an an angle to match the slope of the original foot area.
Here's a top view.
Bottom view showing the braces that both provide the needed height and hold the two top boards together.