Suggestion, try to fabricate it yourself. Start with a piece of cardboard and cut it to size, mark all the holes, set it in just like you would the steel.
Then get a piece of sheet stock and use the cardboard template to mark it.
Since this piece is exposed below you should paint all exposed areas with rust proofing and use some RTV sealant to ensure a gas-tight/water-tight seal. Watch the sheet metal screws so you don't accidentally hit something under the pan.
Of course this depends a lot on what tools you have, but your basic hammer, sandbag, skill saw, and drill should get you going.
Small pieces of steel are available at most lumber yards or from local metal fabricator shops. Actually some fabricator shops might just help you learn how.