I would just glue/nail some short feet on bottom of the front of box, measured to make it level. Or glue the rocks to it so no need to adjust. Rina

Use pressure treated wood. Retreat cut edges with something like Copper Green. Cut right triangles to the appropriate angle to create a level platform for the box to sit. At this point either glue and screw to the box, or attach to the window sill. I'd probably attach it to the box myself, to aviod potential water intrusion to the house through the holes in the sill.

As an alternative, instead of buying a window box, just build one that matches the slope.

As a second alternative, make a few small concrete forms. Tack it to the sill and pour permanent pads (to replace the PT right triangles). You will probably want to dowel it into the sill if you go that route.

PJ

The windowsill is on a slant for a reason. You want water to drain from the box away from the window/house as well. If it was me, I'd allow the bottom to slant flush with the slab, and modify the box so it's vertical at the back and horizontal at the top. If you are building the boxes yourself from scratch, this is much easier. As long as the top if flat and horizontal, it should look fine.

If thats the case, maybe one of these prefabed feet will be more appropriate for you. They will give the box a bit of traction as well. I think that it may work a little better than glueing rocks to the bottom of the planter...HAH!

If your feeling adventurous, and you have a drill, I would suggest the "Tilt and Glide", they are specifically made for slope surfaces.

http://www.componentforce.com/category/514/rubber-feet---plastic-feet

PJ

If you are lifting it level on the bottom be very careful that it drains far enough down the slab so that the water can't creep up against the house or you could be in for a very expensive repair. Try not to put drainage holes any close to the back then you have to. Good Luck!

I know my comment is 12 years late but planters generally sit in front of the cill with the top of the planter about level with the cill. your plants grow up and cover the view of the cill and can be seen from inside the window.