Thanks sofar all for the comments on the G3YXM amp.

I will use a single 58mm 3C85 core as an ouput transformer.
Primairy coil will be using 2mm wire, secundairy will be 1mm wire all PTFE
insulated

The output filter consists of two T225-2 cores, each with about 65 turns
1.5mm wire.

As Dave already mentioned, I will keep an continue air flow trough the
filter.

Not sure what kind of feed-choke I will use.
The 50mm long ferite rod as mentioned by G3YXM or something with ring cores.
(What about a stack of iron cores from old computer SMPS? , those
yellow-white ones, about
 25-30mm diameter?)

Dick, PA4VHF