Making round holes in a cube

Alright, I got a cube, a long and a straight one. I would like to make few round holes in it. How can I achieve it the best way? Boolean difference I thought first. I tried it but i could only make one hole using a tube. Second hole already said that the meshes should be closed. Thank you for your help.

delete one side of the cube
add a circle [or several] aligned with this one side

select the circles and the part where you deleted the face from, and hit shift+f to fill the region

this doesn’t work with subsurfacing [you’d need to work with it a lot, it’d be easier to extrude the face to make a hole that way]

The hole should go all the way through the cube so you could watch through it.

When you have got a plane that has a hole, you can extrude it (with e key) to form a cube. (note: all verts of that plane need to be selected before extruding)

It does fill the circles.

You need to select all the verts before hitting shift-f. If you selected only the circles, it would fill only the circles as in the picture.

How about this method?

The .blend can be found here. Thanks to sh33p for the hosting.