I know you wanted plans for a wooden high seat but to honest I found it as quick to make them out of 1" steel box section. This is one that we made, but some shithouse stole them. I appreciate that not everyone can weld or have access to someone who can weld, but I tried building one in wood and was finding it difficult without using good hefty strong timbers.
This one is a free standing double.
The dimentions of this one are;
Top box section is 1000mm back to front of the padded arm rest.
The seating section being 600mm from the back to behind the knees.
And then 500mm high x 1200mm wide
We also made 2 lean toos, one being the same dimentions as this, but obviously without the back legs and the other one a single being only 600mm wide.
The ladders were 3mtrs high with 300mm wide rungs spaced out at 300mm high.
The whole thing was then bolted together and put together on site.
The 3 cost us a total of about £140 and 2 days labour, not bad i don`t think. By the way, my local steel fabricator cut evey section to size all ready to weld.
We have still got enough steel to make another 3 at my mates place in cambridgeshire, but that`s not going to happen, well not unless i get any land to put them on.
Obviously that was how much the steel was to buy, taking into account we did the welding, drilling etc.
I would imagine that a double free stander like that would retail at about £300??, but for me to build and sell them I would be looking at at least £110, that would take me a days labour inc wood on the seat and a lick of black spray paint.
Where are you in the country? as if you were accessable I may sell you the steel and draw up the plans.