Ironically, it could be precisely because you're trying to enjoy the meringue on a hot day that is causing it to deflate so quickly. Meringue are extremely sensitive to moisture, and a humid day can wreck your meringue's volume. It's best to make meringue on a dry day.
However, there are a few things you can do to achieve better stability:
- Use fresher eggs.
- Fats are the enemy. Make sure you have zero egg yolk in the mixture. Also, avoid hand contact, as your oils can rub off.
- Make sure the sugar is fully dissolved, because otherwise it attracts moisture.
Good luck!
I'm going to take a wild guess at what's happening, based on what I know of thermodynamics, (but I could be wrong about the whole thing)
When a cakes and bread bakes, it bakes from the outside ... as it forms a crust, it can no longer rise. (for yeast breads, this is why we slash the top, so it can get a little extra rise out of it)
With a cake, as the sides set, it'll continue to rise in the middle, creating a domed top. With a metal tube pan, the heat from the outside of the pan conducts to the middle of the pan, so the middle will set shortly after the outside.
...
With the silicone pan, you've actually got an insulator ... the heat's not going to conduct to the inside, so it's going to be able to rise longer than around the inside -- basically, the domed cake, but you've got something for it to stick to, so it doesn't fall back down as it sets.
As I don't think the issue is distribution of the batter, I'd recommend trying other techniques to reduce the domedness of the cake -- cook it at a lower temperature (try 25F / 15C lower) for a longer time. You can't remove the problem entirely, but you might be able to reduce it.
I'd also look at the instructions that might've come with the pan -- it's possible that they have reommendations on how to deal with the problem. (and it's possible that this might not be as significant a problem in convection ovens, but replacing your whole oven for a set of pans is kinda silly)
Best Answer
Consider two options:
Add a stabilizing agent for the compote: cornstarch immediately comes to mind, but I suppose sewit, or custard would do the trick. This automatically lends itself to needing a longer bake time which I can understand to be less favorable.
Try reducing more moisture while prepping the compote. By giving the rhubarb a longer cooking time, you can also intensify its flavor while getting a firmer, stickier compote.