Horrendous noise after sitting for a week...

If the flex plate moves under the bolt head just a fraction they will work loose. You wont hear hear them working loose while its running. The noise appears at restart. Doesn't matter if its a minute or a month. Bottom line... they didn't magically back out while sitting. LOL
Mating of metals, underside of bolt head and flex plate need to be clean and dry.
I have to wonder if the cadmium plating found on reproduction flex plates is a factor in some cases. Regardless... I'll wire brush or lightly sand all those mating surfaces including backside of flex plate and any paint from the bolt blocks on the convertor.
I tighten, loosen and retighten while holding the torque convertor bolts rotated clockwise against the bolt holes in the flex plate.
Lock tight might be easier but... Even if it prevents the bolts from backing out, the flex plate could still move and egg shape those bolt holes over time. Then a knocking noise results at start and shut down.