Then you would have to have closed primaries and registration far enough in advance so that they could draw up the districts evenly.
All of which are good ideas.
Districts could easily be drawn according to the results in the previous presidential election, but it would probably be easier to keep the congressional districts as they are and allocate delegates according to the actual GOP vote in the last election. Some districts would only get a single delegate. Average districts would get the standard three. The remainder would be allocated to strong districts, who might get 4, 5 or even 6.
And, yes, I think closed primaries are a good idea too.