Extra Measures For Storing Coins

Free source of larger silica packets:
Find a local furniture store. Furniture needing assembly before being put on the showroom floor arrives in a box also containing a large silica packet.

Ask a store owner if he would mind hanging up a grocery bag in the assembly area and have the workers save the silica packets for you. Otherwise they end up in a landfill.

If the packets are too large - such as wanting to store a small bag with individual coins - I would think it not too difficult to come up with a packet (such as a teabag?) to accommodate the smaller amount from one of the larger packets.

Get creative. I would think there are other businesses who also toss out a lot of these. My heavy grocery bag full of them is enough for me, and I think that was a three month haul from the furniture store.

All plastics have some level of permeability to both moisture and oxygen, but it differs widely with each plastic.

For example, clear, rigid plastics like polystyrene or polycarbonate (maybe slabs are made of one of these?) are more permeable to moisture at the same thickness, than the polyethylene used in a ziplock bag (of course a slab is much thicker than a ziplock bag).

So I think the idea of using a dessicant inside a ziplok is a good idea.

By the way, dropping a slabbed coin in a bucket of acid is unlikely to harm it (though I don;t recommend doing the experiment). Most plastics are unreactive to acids.