There is a paper slot on the printer compartment so it does not need to be opened for printing.
The wires would simply go through holes in the case to be connected, the rest would be unseen.
There would be tabs/some sort of foam to hold it all in and stop the rattling and risk of damage.
To make life easier, instead of opening up the spectrum, it could simply be placed in the compartment and then removed should it need a repair or something (though this is unlikely!!) Or instead of a DataCorder, the tapes could be put on an mp3 player for beter reliability and smaller size. And because nothing is modded, just put in a case, you could upgrade to a newer spectrum (48k or Spectrum +) very easily!