This is a very interesting question, and the answer to it comes from a simple observation of the following figure. If I am thinking correctly, the correct equation to your wanted rectangle is not unique, because it can be made more accurate by better constants. However for a rough idea, just try to think of this.
The shape of the curve x^2+y^2=1, it is a circle, centred at origin, of radius 1. Basically what we have here is an idea of a metric or a distance function ||A,B|| as the distance between two points A and B, and on one dimension we simply define it as |a-b| where a and b are the coordinates of A and B on the real line.
Similarly we have in two dimensions, the distance between two points of coordinates (x,y) and (p,q) as ((x-y)^2+(p-q)^2)^1/2.
So similarly we can define the equation of a circle as (x^2+y^2)^1/2=|1|, and note that this also gives the shape of the circle only. Now we look at the equation (x^4+y^4)^1/4=|1|. Obviously such a curve would be symmetrical about x axis as well as y axis. Now we observe that such a curve passes through points on the axis as marked in the figure. Also it should be in the region of the green dot, or in the region of the red dot.
Now it is quite easy to observe that the curve will pass through the red dot, because here both |x|,|y| <1 and for a particular x if 0 < x < 1, x > x^2 > x^3 >...
and for 0 < x,y < 1 if x < y, then x^k
In short as we increase n in (x^n+y^n)^1/n=1 where n is an integer, the curve bulges out from the form of the circle towards the square. And as you can expect as you increase n large enough, the curve will approach the shape of the square. Now for any n, large enough you can adjust the curvature of the edges as much as you want.
However I have only exemplified for a square and it can be modified by taking an ellipse with major axes a and b, and a rectangle of the length a and width b. In that case, x-> x/a and y->y/b will give the similar result. In fact you can even get a rounded rectangle as pointed out by yourself, in the comments but this is also a way in which you can get it. Hope you got it.