So sorry Swissotter, been busy gardening and am a bit late to the party, but you've been given a lot of good tips already.
I'd say 3wishes has nailed it perfectly with her suggestions. Also curley has given a good tip about lining the dough, when using fruit that oozes juice when cooking, with ground nuts (not almond flour, that would turn cloggy).
wso I can only add my measly 2 cents
An old Swiss housewives trick, when lacking ground nuts, you can use breadcrumbs or ground rusk (Zwieback) as well for this purpose.
We make our open sweet pies with the dough that picks our fancy or is in store in the freezer...
- short crust (Kuchenteig/pâte a gateau brisée)
- puff (Blätterteig/pâte feuilleté)
- sweet short crust (Süsser Mürbeteig/ pâte brisée sucrée)
The 'Teigli' e.g. custard like liquid, I use 2 eggs, 2-300ml milk and season with vanilla sugar or essence, but for plums... I use about 1/2 tsp of ground cinnamon or my home made pumpkin spice.
Also, we are very fond of scattering an open plum pie with marzipan for baking (this has a softer texture than the one you need for decorations)
When you bake the pie it is done when the top looks golden brown and when you shake the baking dish, and filling with the custard just wobbles slightly but is not liquid anymore.
I wish you success baking and may the pie you'll offer, get lots of compliments!