Hi Tobi,
Plans for Q ships are very difficult. This is why.In WW1 there were 2 types of "Mystery Ships".
Type 1 was warships like Flower Class sloops and PC Boats. Some of them were built to look like merchant ships. There was also the Hyderabad which was a one-off which was designed and built as a Q ship. It should be possible to get plans of these from the National Maritime Museum but they are expensive.
Type 2 were the converted trawlers/coasters/schooners and all sorts. There are, as far as I'm aware, no plans for any of these because they were armed with whatever was available and the Captain had a lot of influence over where guns were hidden and whether he could bribe dockyards to make changes. So, for example, a "normal" coaster could be armed with a single gun, after conversion it could have as many as five or six guns, along with depth charges and torpedo tubes.I'm afraid you'll have to use your imagination but there is a lot of information to be able to get ideas.I'm afraid I've no idea about Grenadier but there must be plans out there but National Maritime Museum have this model
https://collections.rmg.co.uk/collections/objects/67531.html so they probably have proper plans.
Good Luck