I always struggled with the spindle lock button on my cheap router.  Despite stripping it and fettling the mechanism it still pushed out when using a spanner on the spindle.  So here is my solution a simple printed locking screw and ring.  A captive nut 8mm is inserted in the slot, I warmed it to set it in, and a set screw ( mine needs cutting down) and you can lock it easily.

Drawn in Onshape link     https://cad.onshape.com/documents/d896951c8794d3c90d480d35/w/ecf8fc91feb73a413ee0683a/e/37117ec52d16ec02cc9c90b4?renderMode=0&uiState=642c1975ad520607b605d4e9