R Yitzchok Yaakov Fuchs (in his book Hakashrut, p. 665) writes
Baking ovens and toaster ovens that were used for meat and dairy foods
and even for prohibited foods may be used to bake or warm up kosher food on the condition that the food placed inside is wrapped
hermetically in two wrappings.
In any event only kosher baking tins or disposable ones may be used
and not the banking tins and dishes that are inside.
The difference between a regular oven and a toaster oven is that
a toaster oven has limited space inside with the heating element in
the rook of the appliance exposed. While it is in use it tends to get
dirty and it is very difficult to clean completely