This design was for an online contest which asked for a "beach towel holder". Compact and Modular were the only parameters given. this particular design is a blockwhich splits into 4 parts vertically forming 4 "pins" which can be placed on the corners of the beach towel. The ridges on the sides help maintain the towel in the sand.  

The locking mechanism is a simple click lock mechanism which clicks into place when pushed in. The split in the middle acts as a buffer to compress the pins so as to allow them to go in and lock it. This is a cross-sectional view for better understanding