Guitar Footstool
GTfoot 
£0.01 inc. tax
Guitar Footstool Summary
A footstool is an easy almost failsafe solution having the guitar at the right height across your body. There are some drawbacks over time to do with the possible twisting of the back and stress on the knee. However, it is the most popular solution.
I am recommending the product below (Amazon associate Link) because it has 6 height postions. Many footstools only have 4. It can therefore be more finely adjusted to the pefectd height for you regardless of the height of the chair you are sitting or how long your legs are.
Please the youtube video below the link for a more detailed explanation and opinion of the pros and cons of using a footstool.
I am recommending the product below (Amazon associate Link) because it has 6 height postions. Many footstools only have 4. It can therefore be more finely adjusted to the pefectd height for you regardless of the height of the chair you are sitting or how long your legs are.
Please the youtube video below the link for a more detailed explanation and opinion of the pros and cons of using a footstool.