Help me cut my shape

can anyone help me cut the inside lines out of my shape so that there is a groove going through it thats the depth of the sqaures

heres the file

oar clamp12.skp (355.5 KB)

Like this?

oar clamp12.skp (355.5 KB)

the file is the same as the one I uploaded

Sorry must have chosen the wrong one. Here it is: oar clamp12-amended.skp (358.2 KB)

I started by exploding everything back to raw geometry. Then I push/pulled to create one half of the model. Then I mirror copied the other half and deleted the join lines.