Post a picture of this cabinet, so we know what you are talking about.

Cutoff waveguide says that the holes in the cabinet must be larger than 1/2 wavelength (electrical width),
hence 11.8"/2.54 ghz is the wavelength = 4.65 inches, so you'd want holes in the 3 inch size.

You can use the same cabinet and same top antenna, you only need to couple the energy off the laptop antenna and send it thru your present antenna on top of your cabinet.
Laptop antennas are not identical, but they are close to being identical if they operate at the same frequency. You can have a cable with a coupler (just a wire) that you place on your laptop antenna. To find the optimum wire shape to couple off the laptop antenna, you need a little experimentation. Start with just stripping a cable to leave about 3 inches of center conductor exposed. coil it in a circle to fit the typical laptop antenna size, and place it on the laptop antenna. Then cut the 3 inch length down to 1.2 inch minimum (1/4 wavelength) to find the optimum size, maybe changing the shape too. Encase the wire loop in a plastic piece and make a clip to attach to the laptop antenna. You'll probably only lose 2 dB in the coupler, which isn't much and will be better than making holes in your cabinet.

kch
If it works well, send me a dollar.

The hard thing is preventing emitted or received RF energy, not letting it in. Slots of half wavelength will do fine and you need both orientations in order not to give an attenuation due to incorrect polarisation. There is no need for a complete hole, a slot will be equally effective.

One trick would be to use a metal side that was insulated from the other sides (including plastic screws or insulation kits). Alternatively the screws should be half a wavelength apart. As a starting point, the insulating gap would need to be at least 1mm.