It's not my idea. Credit the Extreme Macro website for this.
http://extreme-macro.co.uk/reverse-lens-hood/
You have 3 tubes so you can adjust the length. The shortest tube is usually around 5-7mm. If you look at the design of the old macro prime lenses the front element is well recessed within the barrel of the lens. On my Super-Multi-Coated Takumar Macro 50mm f/4 (screwmount) the front element is recessed around 50mm from the filter ring. I've rarely have problems with the barrel getting in the way even at 3.5x extension on my bellows. When the front standard of the bellows is reversed the lens is recessed. I have more problems with the rail getting in the way with the object than the standard of the lens. The standard has a 52mm filter thread so hoods or filters can be added.
Ambient light hitting the lens element can cause flare and reduce contrast.
From Extreme Macro:
One of the most common issues with reversed lenses is overall greyness of the final image and lack of contrast. Adding a hood is a quick fix, a cheap way to prevent excess light from straying into the image.
Extreme Macro Greyness Problems
With diffused flash macro photography, we like to have a nice large flash diffuser that covers a lot of the area from the subject's point of view. The problem is though that if light enter the lens directly from the diffuser into the lens, ie not bouncing off the subject into the lens as such, then this just registers as a uniform grey fog and gives you a low contrast photo with a bit of a washed out look. To some extent this can be corrected using levels in photoshop by adjusting the middle slider, but it can also show up as a much more difficult problem to deal with, a grey or light spot in the centre of your photo. Levels let you adjust the entire photo, but dealing with a circular highlight is a bit trickier, and the better cure is prevention.
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And as pointed out previously take a plastic end-cap and cut a hole it the bottom. This will protect the rear-end of the lens and also act as a lens hood.