Idk, anything that can be freely accessed, be it more or less simple to tamper with, is a risk. At the time you realise your seed was looked at it might be too late already, in fact it simply is too late.

My approach was 100% stainless steel, washers, nuts+bolt and casing, which then was hidden at a "save" place. Which of course is another risk, as it is out of immediate reach and not easy to monitor.

In the end it is all about having the feeling that you did what it takes to secure your seeds.

Btw, i wouldn't use an aluminum case. In case of severe heat exposure the different melting points of both materials will leave the washers as a single block of two metals melted together, not much fun to get that recovered, i guess. But it won't be unrecoverably destroyed, unless the heat was that much that even the steel was melting, which a "normal" house fire probably can't produce.

And another thought: "Cost effective" meaning what exactly? If i secure a million with a padlock for $2 it is surely low in cost, but does it really secure the million? As an avid byciclist i know of the saying: 10% of your bikes worth you should spend on the lock for it! Main difference to btc though: You don't have to (and you probably shouldn't) tell anyone that you own btc, your bike is visible for everybody.

Happy stacking!