The best idea is like someone else mentioned above and that is to make a portabel running-in bench and drive to a flying field to run your engines. The idea of running-in a new engine is nice and exciting but if you have modern engines longer running in is not necessary. Just bolt it to your planes, run a couple of fuel tanks thru them , for about 20-30min, and after adjusting mixture and main needles the engines should be ready for flight. Thats what i have done the last 10 years and worked great every time.
Running the engine in a garage is never good as it blows the dust and dirt around, makes to much noise no mather what you do and the engine can suck in dirt which will shorten the lifespann.