how to design a testbed

Could you give me any advice about the design and mounting of the propfan ground testbed?"

Yes. Do your homework. Model it in 3D CAD. Analyze the results and design accordingly.

Who will you ask when you have a deadline in a real job?

Never mind the chickens.

Did you Wiki: "design and mounting of the propfan ground testbed?"

The question is just too broad with the information given!

Can you provide more information please including the size and type of the propfan, if this is a theoretical or practical exercise (if practical then - budget, actual tests you want to conduct (vibration, prop balance, full thrust running test, etc, etc)).

Also, what do you have so far (I am assuming you have done a basic search of the internet)? You have at least done this basic and obvious check haven't you before asking here?

More information please.