Kevin,
The top to my dutch oven was aluminum but it was
sheet, not foil. If you use foil, you will probably
need something to reinforce it in the middle so it will
not sag from the weight of coals.
I suppose really heavy duty foil might work,
but some kind of span might be needed.
Instead of aluminum sheet, I can think of 2 substitutes
1)flat after-market aluminum splash-guard cover for
frying pan.
2)larger cake pan, inverted so it locks around the original
cake pan.
Sometimes these items are found very cheap at discount
stores, or thrift/charity second hand stores.
Another, consideration: if you put the pan directly on coals
it might scorch on the bottom, even if 3/4 coals are on top,
because the direct contact on the bottom causes intense
heat flow. You could slightly raise it, with small stones or
a grate, or change it to 5/6 coals on top.
That is where the art, the fun, and good experiences begin.
Bring extra batches of bread/biscuit fixins.
When you eat your first good batch, laugh a little, because
you are there and we are notl