Dave Rutherford tried a GRP bonnet some years back, without success.
If you get a short inlet manifold and stubby trumpets, you might find it will fit under the bonnet.
Talk to Helen Thorne (at Richard Thorne's), she races a 1964 4/4 with twin carbs and I am sure she has the standard bonnet.
Otherwise Vintage sheetmetal will add an intake for you in steel/ally to your bonnet or supply you one to cut-in/rivet etc.