If you are making a belt drive you want to mount it on a single solid power plate. Pay special attention to the structure of the length of the plate running parallel with the belts. It may be fine when sitting or not under load but once you pull the trigger you may see the plate flex and the belts will become to be loose. This causes a lot of vibration and damage to your equipment. Reinforce this direction with either angle iron or bends on the plate to add rigidity.
After you have built this power plate you can mount it on
rubber feet for vibration dampening or if you plan on attaching it to another frame use the vibration dampeners that have bolt coming out of the top and bottom with rubber in between.