I've seen this being done before (I think on LAF) at least with other playmobil ships. After finding some examples on other people's blogs I came to the conclusion I really needed such a ship myself so my 28mm frogmen would have something interesting to assault.
I managed to pick up a second hand ship, albeit with a few parts missing.
Plasticard was used to cover the cargo hold and the large door openings. Other bits such as WHFB shields, life belts, 40K rhino searchlights and bits of sprue were used for detailing.
As with all the things I build, playability is more important to me than historical or real world accuracy. It has to look the part. That is why I put all the bits against the sides of the wheelhouse, leaving room in the centre of the wheelhouse to place figures.
More pictures and details on my blog:
https://toomuchterrain.blogspot.com/2021/04/the-titan-converting-playmobil-ship-to.htmlT.