I would use styrene sheet for the various internal bits and then you can glue it with “ordinary” poly cement. or superglue, or plastic magic, whatever takes your fancy.
Other than there being a thread on this elsewhere, as it must have been done before, I would go about it by first seeing how much stuff the hull will carry (before it goes well beyond the waterline)
You will need to fit in r/c reciever, mini servo, motor (speed 280 more than enough, or perhaps a pair of 280s) battery, ESC and shaft/prop/rudder assembly, motor mount, coupling(s), wire, rudder/servo rod.
You will need access, so the main portion of the superstructure needs to be demountable, possibly on a combing, or pins….you need access to the rudder tiller,
How big is the boat? are you having 1 , 2 or 3 props? 2mm shafts will likely be needed due to the weight problem but there is not much choice of props in this shaft size.
Ashley