Don't know if anyone has already described this but this is how I do similar holes etc.
Open the component into which you want to make the holes for editing. Draw the surface shape, i.e. your rectangle, onto the surface where you need the first hole. Double click and make the rectangle a component. If you holes need to be evenly spaced along the length (with me that is generally the case) use the "Move" tool, tapping {CRTL} to make a copy and move the rectangle to the position of the furthest hole and click to put it in position. Now type {/} and the total number of holes in the piece minus 1; for 15 holes you would type "/14" and hit enter. Voila, 15 evenly spaced rectangles.
Now double click one of the rectangles to edit it, and use the push/pull tool to push it as deep as it needs to be. All the holes in the the piece will appear as you work on the one.
Edit in> A little more needs to be added. When you do the push/pull it will create the void you want under the top surface, but that surface remains intact. So, watch the depth on the VRB or PUSH/PULL then enter your desired depth into the VRB. After you click or hit return, click on the surface of the rectangle you're editing to hilite it, and hit delete. Then hilite the surfaces inside the hole, right click and choose "Reverse Faces"
Here's a Tut:
http://www.sawdustersplace.com/Sketchup%20files/MultipleHoles.skp