One of the best fittings for the job is this new ball lock post cap. This attaches using a sanitary 2inch tri-clover fitting and then couples with the neck of the commercial kegs such as the 50L kegs.
https://www.kegland.com.au/ball-lock-tapping-head-to-2inch-tri-clover-commercial-keg-adaptor.html
This is a new fitting and as far as I am aware we are the first to make such a fitting and might work out to be better value than having to purchase a keg coupler and the ball lock post adaptors.
I would imagine this might result in some customers also using the 50L kegs as fermenters as they are the cheapest stainless pressure rated tank that you can purchase. I still personally feel that the kegmenters like this are probably better as the opening is a bit larger making them easier to get your hand in for cleaning if necessary:
https://www.kegland.com.au/29l-kegmenter-304-stainless-uni-tank-pressurisable-fermenter.html
https://www.kegland.com.au/58l-kegmenter-304-stainless-uni-tank-pressurisable-fermenter.html
With that said if you are wanting to save a few bucks this would achieve the desired result. I can also see some home brewers who like using the ball lock fittings but want to do larger 50L batches then this also might suit that type of customer as well.