Hi Juanman,
I too have converted a 4912 to be mounted under a countertop.
I have mine mounted under a bar made out of standard base cabinets. The width of the opening is 24". This allows for about 1.5" on each side and the top with several inches in the bag for airflow. I have not had any problems with the compressor overheating but I have only had this setup for a few months and summer is stil ahead!
I cut a 3" opening in the countertop and a 3.5" hole in the top of the sanyo. The 3.5" hole allows you to slide 3" (inside diameter) PVC up throught the hole in the 4912 and pressit up to the bottom of the countertop.
BTW before you do this you will want to mount some T-nuts under the countertop to mount the tower to later. I also used R19 insulation with a 3.5" hole cute in the center to slide the PVC through to keep it insulated.
After I have the PVC in place I used metallic duct tape to seal up the top and bottom and make sure everything is air tight.
It has been working well so far but feel free to ask if you have any questions.
Pics are here.
Flickr: Photos & Video from OlathePin
Good Luck,
John P.
Semper Perfidgus Fermentum