An inertia dyno works by measuring the acceleration of a heavy drum. The more powerful the engine, the faster the acceleration will be. This is a very common form of dyno, and extremely simple in its operation. The main issue for a DIY’er is to get hold of a suitable drum. They typically weigh several hundred kilos.
YourDyno.com’s data instrumentation kit and software support inertia dynos.
Some inspiration for drum making
Here is a link to a guy who made his own inertia dyno.
Get some inspiration there and build your dyno! Or see Brake dynos options.