There are fixtures available specifically for riveting the trigger guards. If you don't have access to a shop press it will not do you much good.
What I would recommend is to use a plate inside the receiver where the 4 rivets would attach the trigger guard at the mag release. 1/8" flat stock works well.
Essentially size the plate to be the same footprint as the trigger guard. Transfer the hole pattern to the plate, drill and tap as required. I can't remember if the socket head cap screws used were #6-32 or #8-32 x .25. You may need to chamfer the sides slightly to clear the inside radius of the receiver. You may need to notch the plate in the area where the mag release is. Essentially match the contour of the selector stop and you should be good.
If the heads of the screw prevent fully tightening them to make full contact with the trigger guard, you can reduce the head diameter with a file and a drill. Chuck the screw in the drill and file until fit is correct. Use some cold bluing to touch them up. Degrease all threads and apply red loctite on all screws then tighten securely.
For the rearmost hole on the tang you can use a nut or get real fancy and use a PEM self clinching nut that is pressed into the sheetmetal.