First flush system for European drains

Hi y'all
We are looking for a first flush diverter for our rainwater harvesting system.

When we first moved up here to our homestead we were both very eager to make everything ourselves, and also the - mainly Australian - systems that we found online cost an arm and a leg and didn't fit our European drains... Now I can find a lot of systems that fit the European drains, but they are all in the US and I would prefer if  I didn't have to pay toll on top of the price (also because paying toll when importing as a private person is a hassle...). So 5 years ago we built our own - see attached image. It kind of worked, for one winter - then the brutal sun in Southern Spain destroyed the black rubber at the top.

So we are looking for a new diverter, and we are willing to pay for it - because frankly we have actually payed quite a lot for what we have now, which doesn't work. We would very much prefer to have one that fit our drains though. We would also prefer one that people have experience with and knows works (we don't want to repeat our compost toilet experience). And we would prefer one that can handle the sun in a mediterranean climate. And we would prefer one that doesn't cost an arm and a leg.

Is that doable?

Can anyone help us?

Cheers

That black rubber thing looks like a bog standard toilet floor fitting and costs abiut 3 euros. Any reason you cant just replace it?

If the rubber was the only casualty of the summer I suggest you find a way to shade it.
BUT, if you look closely at how the Australian first flush units are made, you may be able to create your own anyway.
PLUS, if you study how it works there are different ways of achieving the same thing.
I recently helped via Kickbooster to publish a book on water collection.
Some of the techniques were amazing and simple/
For instance, a first flush system used a tipping bucket, to capture the first flush, dump it and let the clean water go to the tank.
Perhaps look for it.