The engineers at IKEA are geniuses: they produce these bulky light and surprisingly sturdy items for way less than you could make at home. If you don't believe me, then try building this
for 120$ (well you actually have
to build it either way...). They make some of their furniture seem bulkier than it seems by literally filling it with air. For the LACK series they use corrugated paper to provide some stiffness.
CAUTION: You will be using power tools here, it is always better to protect yourself rather than spend half a day in line at the hospital (for you fellow Canadians) just to get a piece of sawdust out of your eye, or stitches on your finger. Use appropriate safety gear and be careful.
DOUBLE CAUTION: This shelf is build very cheaply meaning you will easily scuff, chip and buckle it. Pay particular attention when retracting the jigsaw as the blade will continue to reciprocate after you let go of the trigger and could do some serious damage to the finish. I ended up turning the brackets around because I had chipped the top side of the shelf.
Before drill you should remove the sticker from the back of the shelf and save if for later.
The goal is to make a very clean rectangular hole in the back of the shelf. We want to leave enough material on either end so that the wall brackets are very secure, and we don't cause the shelf itself to collapse. I centered a gap of 5 and 1/4 inches. To start, drill two big drill holes at the extremities of your planned opening. Then use your jigsaw (use a long and fine blade if possible) to cut the hole to your plan. Try to get the cut as square as possible because you want the safe door to be snug as a bug. I ended up using the jig saw fairly aggressively to scrape some of the particles and glue from the inside of the shelf; again be very careful to not scuff or chip the finished surfaces.
You can use a pocket knife or screwdriver to cut the corrugated paper loose from the top and bottom of the shelf then simply push the paper down and to the sides with your hand to make the hole the desired size.