I struggled with this conundrum when I first acquired a Hasselblad. I ruled out a proshade as being too big and unwieldy. I bought a Bay 50 (I have the older C lenses) to 55mm adaptor and used Cokin filters but I wasn't happy with that. In the end I bought Bay 50 Hasselblad filters and hoods for my 80mm and 150mm lenses. The hoods fit onto the filter and it's a much neater solution. If I want to use a filter other than what I've got in Hasselblad fit I can use the adapter and Cokin filter instead. I also use Cokin filters on my C Distagon 50mm, but that's another story...

Hasselblad filters are very expensive, but if you're patient you can find reasonably priced ones on Ebay sometimes. The Milo and Aroma filters tend to be cheaper than Hasselblad filters - I don't know if there's much difference in quality. I bought a Chinese Widepan hood for my 80mm. It's plastic but quite sturdy and fits perfectly.