Hi, Kim. Yes, a madeira cake would be your best bet. It's denser than a sponge, so will better support whatever you want to put on top. I don't personally think you'll need to freeze it - a madeira will keep easily for 4 days or so, so you've plenty of time for decorating it. Not sure about the amounts needed for your size of tin, but I made a Night Garden cake for my DGS1 last year and this is the recipe I used for the bottom layer, a round 9” orange Madeira cake, 3” high (sorry about the imperial measures - I'm old school!).
12oz softened butter/margarine
12oz caster sugar
1lb plain flour
2.5 teasp baking powder
6 eggs
2 tblsp lemon/orange juice
Orange rind
Gas mark 3, 1 hour and 30/40 minutes. Grease and line tin.
Cream butter and sugar.
Sift flour, beat in eggs 1 at a time (spoonful of flour after each). Fold rest of flours in, then rind and juice.
Into prepared tin, hollow centre.
Cool 5 mins in tin, turn out.
I made a couple of pirate cakes for DS back in the day. I'm sure you have plenty of ideas, but rice paper slotted onto straws made excellent sails, licquorice allsorts came in handy for cannons etc, and I still have the Lego pirates that strutted the decks!