Not sure of your 3d printing background, but the Almond+ should not be too bad. Depends on 2 things:
1) Do you want to cover the entire router (more complicated) or just a portion (just front/sides or front only?
2) How big is the print area of your printer? For example, mine has a 4"x4"x4" area (on a good day) as it is an "old" Thing-O-Matic.
For larger objects than your print area, you need to design the object in pieces. The most common is to make the connecting sides have simple puzzle-like edges. Or for vertical additions you use a post/socket to connect them together. If needed (depending on fit and tolerances) a little super glue (or other depending on material) would seal them together. Then there are things you can do for a smoother look.
Barring the 3d printer method someone could always paint their Almond+ or put a veneer on it... Although I would think those 2 would be instant warranty voids.
Most important aspect, be careful of openings. The Almond+ needs airflow. Plus you need to keep the touchscreen and ports clear.