[Alavinus]

If you have the space you can take a piece of delrin and drill a hole off center. Then when you want to tension the chain you can just turn the Delrin. The cam action will tension the chain.

Another option is to use an idler sprocket or another peice of Delrin attached to a rod and place the rod in a slot that you can move back and forth. This too will give you tension.

The third way that I can think of is to think of a way to move your motor mounts up and down. This gives you great tension. I have been on a team that used this with great success.

The reason that I recommend Delrin is it has a very low coefficient of friction. This means you don't have to make it roll to deal with the chain.