Sitand

I suppose I could have spelled it "seatand", but the way people use (or mis-use) English, I figured I might as well be first to shorten the word and retain the sound (there are other words where "i" is pronounced "ee"). At least I'm not violating one of the rules, where a vowel, when followed by a single consonant, is supposed to be pronounced as a long-vowel. Others, of course, will pronounce it as if it was spelled "sittand" --but since it's NOT spelled that way, it shouldn't be pronouced that way. The purpose of this paragraph is to point out little details like that (semi-proper English).

Anyway.... The following descriptions are for a basic version of the gadget, suitable for the average person (who doesn't have to turn sideways to go through a door). A wider version would be needed for wider people. Do note that the average standing human, if observed from the side, is mostly flat along the back side. The buttocks stick out some, and this gadget is designed to accommodate that.

Start with a short wide plank, maybe a meter and a half long and half a meter wide (3:1 ratio). Build a height-adjustable framework to hold it off the floor at perhaps a 45-degree angle (tilt should be adjustable, too). Cut the plank about half a meter from one end. The two pieces are now 1:1 and 2:1 in ratio of length to width. The larger piece we mount on the upper part of the framework, and the smaller piece we mount lower-down on the framework with a gap of about a third of a meter between them. We want to make sure the framework does not nearby-occupy the space between the two pieces. Without portraying the angle, here's a sketch of the relevant lengths:
---..------ (periods used to represent spaces)
--| At the top edge of the lower piece (pointed at by the vertical bar at left), we make sure it is well-supported and well-cushioned. Lesser cushioning is applied to the upward surfaces of the two pieces, and a head-rest is added to the top of the upper piece. Now it is ready!

Stand beside the gadget and, using the height-adjustment, position the gap so it is just a little lower than your buttocks. Turn so you can squat just a bit and fit that part of yourself into the gap, as you swing the your legs over the lower angled padded piece. You are now sort-of sitting on the strong/padded upper edge of the lower piece, so you don't have to have much weight on your feet as they touch the floor. Without the padding and that angled piece of plank, it would be like sitting on a metal cross-bar, but this is intended to be comfortable. Lean back. The head rest allows you to look forward (instead of upward at an angle) without you using your neck muscles. The lower piece shouldn't be longer than the length of your thighs, allowing the knees to bend and the calves to descend vertically toward the floor. The mathematics of "physics vectors" ensures that a considerable part of your weight is supported by the padded pieces of plank, not by your buttocks. You will be more comfortable than you might first expect.