I usually do it the way that you are doing it, because they move on me too. Some wooden shop made T squares and sometimes a clamp or two to hold it in place. I love pocket screws, but hate their appearance, so I will only use pocket holes and screws where there's no way to see them from the outside of what I'm building, or when reaching into a cabinet. I'll usually use biscuits, glue, and clamps or dados, glue, and clamps where the pocket holes and/or screws could be seen, but pocket screws sure make assembly easy for me when I can keep them hidden in the design. Oh, I hate the pocket hole plugs too.
Right now I'm building burial urns. The mahogany wood that I'm using is only 9/16" thick and not 3/4" (from my source that size). The 4 sides are mitered and splined at the corners, but the top is attached using the Kreg mini pocket hole jig to drill a pocket hole in the center of each side and a stainless #7 round head screw is used in each of the four sides, from the inside (Kreg doesn't offer 3/4" #7 fine or coarse thread stainless pocket screws) to attach the top. I use the same stainless screws countersunk into the bottom to attach it into the sides. The bottom can then be removed to place the cremains, but no fasteners will be visible when the urn is fully assembled and sitting upright. The urns are being stained and finished like fine furniture, but they should survive well if ever placed in the ground.
Central North Carolina