It sounds like you might have Dutch ovens and not camp ovens, you could try using a cake tin base on top of the lid to keep the ash out

I'm a Canadian. 

(All of the below is talking as if you might not have a Bedourie oven  http://www.southernmetalspinners.com.au/camping-page.html)

North Americans call the kind of cast iron pot that you use with coals below and above Dutch Ovens.  The N.Am. oven typically has legs on the bottom and a straight rim around the edge of the lid.  The lid is also flat across the top.

There are also pots with no rim and no legs.   One can use them with coals above and under the pot if: 

a) you use foil or a cake tin sides to create the coal holding rim &
b) a lid stand or trivet with 1" or more legs. under the pot.

The current manufacturers call all of these pots  Dutch Ovens as long as they are wider than they are tall.  Go figure. 

In N. Am. the older ones are the ones with the rim and the legs.

& the rimless legless pots are the ones that came in style after cast iron stoves became available.

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Chuck wagon ovens is a term for the legged  and rim oven because they who traveled with chuck wagons used them

& as I said, this is all leaving the Bedourie oven out of the discussion.

Thus, I'm trying to say, what I think you have are newer than Aussie camp ovens or N. Am. Dutch ovens.

Some years ago I made a retainers out of 25mm flat bar, for three ovens of varying sizes, they work well for me.

Try a used foil pie dish of appropriate size

you may even have to buy an apple pie

push it down over the centre bizzo