The Deployable Practice Target can be described as an inflatable starship.
The device is made of a triple monolayer of materials starting with an organic polymer designed to keep gasses inside, followed by a psudofluid sealant layer, and finally a metallic film which reflects sensors and small metallic patches that reflect light to create the illusion of on-ship lighting. Since it is deployed in space, it only needs 27 cubic meters of compressed dry nitrogen to inflate fully. Deflated, the Practice target is about the size of a desk and can easily be deployed from the proton torpedo tubes once primed.
Deployable Practice Targets are not very sturdy structures and will pop under enough force, such as being rammed by a Sulek-class surveyor vessel like the USS Jefferson Randolph Smith. (TOS novel: How Much for Just the Planet?)