Here it is...
Make a tube from a black material, attach to flash head:
http://farm2.static.flickr.com/1016/1483896324_8e8333d5bf_m.jpg
If you want the light to be more focused (minimal side leak), make this light concentrator by using black straw or plastic board:
http://farm2.static.flickr.com/1425/1483897872_91515a1e91_m.jpg
Flash with light concentrator, minimal light leak:
http://farm2.static.flickr.com/1078/1483043131_5d0d75ea53_m.jpg
Light come out from Flas with snoot and light concentrator:
http://farm2.static.flickr.com/1234/1483043619_9a6f14e480_m.jpg
DIY Flash diffuser (Tracing paper as a diffuser), inside box lined with silver paper for a more uniform light:
http://farm2.static.flickr.com/1119/1483043825_805cb7dde8_m.jpg