Have you inlet the trigger? I always do that after the lock is inlet to make sure the trigger and lock function correctly together. How have you determined there is no room on the trigger plate for a tapped hole?
As others have suggested, there should be room on the trigger plate, if not, may be best to get a new trigger, as this is a new build (correct?), and you want to get it right. Others have asked, but I’ll ask again, could you please post a photograph of your trigger? It is unlikely that it is so unique that no one here has not seen or used it or a similar one. Below is a photograph of three triggers on my workbench. Does your trigger look like one of those? The two on top are intended for TCs and there is no practical place to drill and tap in to the trigger plate. The one in the middle has a clearance hole in it to allow the tang screw to go through it and thread into the trigger guard. The trigger on the bottom is new and never install or fit in a gun. It has a trigger plate similar to what you should have. Plenty of room for a threaded hole and extra length to so it can be trimmed to whatever length works or you desire.
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