Small (cheap) manual flash?

I'm looking for a small cheapish flash to use to take photographs inside bird nestboxes (birdhouses). I want this to record the prey that parents are bringing to the chicks, so the lighting doesn't need to be anything more than functional. The working distance will be about 25 cm and the conditions will always be the same, so the camera + flash can be set entirely on manual. Because of space constraints, it would be helpful if the flash fired along the long axis of the flash unit (that would usually be +90 degrees). The flash unit also needs to be small. I will (probably) be using a Nikon D3100 camera.

Any recommendations?

Would the Nikon SB-N5 work on the Nikon SLR?

Many thanks.