For me, something like a merchant carrier would be better than a full carrier. If only because they have experience with Armed Merchant Cruisers, so they already have some background for understanding conversion process and crewing needs. We're then looking mainly at scouting and limited attack capacity for the aircraft which the float planes will be suitable for. The development needed is then how best to recover aircraft, while the outline attack plans could potentially be worked through with Japan - provided they can offer something that the Japanese want [1].
The other thing is that building or trialling an actual carrier is going to get the RN interested. I'll believe it's possible to prepare for a merchant carrier conversion on the quiet, but I can't see how it would be possible to hide constructionand then trials of an actual carrier.
[1] fuel - Germany doesn't have much to spare, technology - maybe aircraft cannon, 37mm AA, and tank designs? But then does Japan want to admit to a lack of capability and if so, do they have the industrial capacity to build them. Plus the Panzer 1 and 2 might be quite good in the mid 1930s when naval trials would be underway but less so by 1940. And there's still the question of industrial capacity
The other thing is that building or trialling an actual carrier is going to get the RN interested. I'll believe it's possible to prepare for a merchant carrier conversion on the quiet, but I can't see how it would be possible to hide constructionand then trials of an actual carrier.
[1] fuel - Germany doesn't have much to spare, technology - maybe aircraft cannon, 37mm AA, and tank designs? But then does Japan want to admit to a lack of capability and if so, do they have the industrial capacity to build them. Plus the Panzer 1 and 2 might be quite good in the mid 1930s when naval trials would be underway but less so by 1940. And there's still the question of industrial capacity