Asphalt roof sealant would probably work, that stuff sticks to anything.
And yep the expanding foam, as much as I hate the stuff, would stick to the block well I think ... not so sure about the plastic.
To bad you are not my neighbor ... I have 20 tons of 1/2" crushed stone in a pile on the property, got for a project and never used it.
Heck, one simple solution would simply be rocks or bricks along the perimeter, it would take care of 90% of the problem. Put them as close to the inside of the block wall as you can. I don't think the amount of vapor escaping around the edges would be an issue, the vast majority will be coming from the 600 square feet of dirt. It's not like the vapor is under pressure and will seek out the edges.
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