Squaring board edges is one of the typical job for traditional woodworking hand planes where they show their effectiveness. Stanley produced the famous but rare 386 as help to be applied on side of jointer planes.
Of course best modern plane makers offer similar solutions. I recently found myself having to do the squaring job on long fir boards, using the new Stanley 62, a low angle plane with the length of a jack which reproduces the historical N° 62. It is not a jointer but the low-angle configuration offers advantages in softwood planing. The sides are very low, not allowing the anchoring of traditional guides. That required a self-built solution. So I made a simple guide with a beech board (composed of several pieces assembled for greater stability), reproducing in the upper portion the shape of the plane and leaving a protrusion from the bottom of the plane of about 7 cm.
Another removable piece, anchored to the main board, covers one of external parts to the mouth ; so, when you join the guide to the work piece you are within the cutting area of plane. This piece could be replaced in special situations with a shaped board in order to have a different angle than 90 degrees.
anchorage system consists of two L-plates with a central pivot to create a very effective third type lever in wich the grip is guaranteed by rubber interposed between the plate and the plane.
During processing, the main strength is carried laterally to held the fence tightly in contact with the board. The knob is used to improve lateral handling. And here it is at work.