A plunge router is great for cutting mortises. But you need a jig to hold the workpiece and guide the tool as it cuts in order to get the precision required for a mortise. This versatile jig allows me to adjust the length and location of the mortise plus I can use it to cut mortises in angled workpieces. To use the jig, place the piece to be mortised against the upright face, clamp it in place, and then adjust the opening as needed. Use a plunge router with a spiral upcut bit to cut the mortise in several passes. I find that a spiral carbide bit cuts fast, and makes a nice, clean mortise.
The heart of this jig is the two-piece platform. It provides a wide surface to steady the router base. It also incorporates stops that let you change the opening for different mortise lengths. The adjusters allow you to replace the stops for different size guide bushings, in this example 3/4". The stops are held in place by strips of hardboard which match grooves cut in the two platform pieces and lock down with threaded knobs. The top bracket holds the platform in place and the bottom bracket provides a place to clamp it to a bench. The upright is slotted for clamping the workpiece and to clamp a guide for making identical mortises in multiple pieces. Assemble the jig with glue and screws.
The side view shown here provides more information for building the jig.