There are some odd idiosyncrasies when it comes to building a GP (groundplane). If you have a 1/4 wave antenna (which you do not), the design is for the radials to come off at a 45 degree angle from the base. Changing the angle can change the impedance of the antenna and therefore the SWR.
Now, a 5/8ths wave (closer to what you have) generally is built to have the radials come out at 90 degrees to the base (flat). This is a little more difficult for you in your situation.
If I were you, I would put 4 radial wires on the base and drape 2 along the length of the window sill, put one inside the window and the other outside, just hanging. This will be less than optimal, but it WILL work.
The best method would be to have the antenna mounted in the center of a solid, flat piece of metal at least 38 inches across. 1 radial wire does help things. 2 is better, 3 is better still and 4 is great. IMO, anything more than 4 radials is overkill unless you can mount to a solid piece of metal.
WM