Used these solid cable entry plates in a couple of speaker cabinets and about to use one to replace the port on a 2x2 Marshall.
The plates are quite large compared to standard circular recess, but the 5cm projection into the cabinet isn't really a problem for 12" speaker boxes.
The holes match standard Speakon connectors exactly and need only a couple of self-tapping screws each. I favour the combination 1/4" jack and Speakon connectors.
The big advantage is that the cable plug is entirely within the 45 degree cut-out so doesn't stick out to be knocked when moving the cabinet around.
A slight downside is that there are 2 holes so you'll usually need to make-up a masking plate; it would have been great if one hole was a knock-out or it came with a blanking plate.