I wanted a pocket-sized, interchangeable bit driver set, but was faced with a dilemma: 'proper' versions are never pocket-sized, and pocket-sized versions are, without exception, crap quality. So I decided to purchase a high-quality set of bits, plus an extra bit-holder, with the intention of making my own pocket-sized, interchangeable bit driver set. This 'thing' is a simple mold that allows you to form a solid handle onto an extra bit-holder using a high-density castable (e.g. epoxy putty). The 'blind' hole ensures that the bit-holder shaft lies down the centre of the mold. The 'open' hole will need to be drilled out to match the diameter of your bit-holder socket. Lubricate the mold with silicon spray (WD-40 or CRC should be OK, too). Once firmly packed with castable+bit-holder, clamp a piece of plastic onto the open side to ensure a good finish. Once cured, cut/break the mold away. The finished article fits into the set of bits, and can be used directly or extended with the bit-holder from the set.