the Targotech adapter is useful if the Sigma is not a Nikon "G" lens equivalent where the aperture is only controlled electronically. If not, the Sigma lens cannot be mated with the adapter cause it will only shoot at the smallest aperture by default and you cannot control it in any way.
16:9 and Novoflex sell nikon G lens to Canon adapters which allows physical control of the aperture. ( and it is much more expensive )
Why don't you just get a Canon mount lens instead? A sigma on Nikon mount should be about the same price as a Canon unless it is truly rare or you have an emotional attachment to it. In that case, the price you spend on a second hand body of the same quality of your 300D might be the same if not cheaper.