I believe steve's injector was behind the flameholder but directed forward into it, not ideal but it worked for him.
Honestly ramjets IMO are a bit of a black art, truely only quantified by proper testing and development.
However a few basic principals and parts remain true.
A ramjet can be built to any size but I would still follow guidelines set out by Becker, but then as your target speed increases things would need to change, your ramjet will need to be longer to allow more time to slow oncoming air etc.
Inlet spikes are always fun but hard to manufacture, but the biggest problem i've found is generating enough air flow to run the things
Maybe get a solid block of balsa, and cover that with an epoxy with a high temperature hardener like 2404 (Araldite product) or 5163, and add some talc to it as a gelcoat. Because its behind a pj, its gonna burn. Only metal will really hold up to it, but have fun forming a nice curve with it. Maybe you can find some pre made stuff online that will do the job? The balsa will give you something to screw into to hold the thing in place.
On my large engine, I welded a bolt to the flame holder, so the spike would thread on, then could be located with some bolts going through welded on nuts up the front.
you made a Talon? Link me? A spike is an aid to change air pressure on its path to the flameholder, also provides a nice way to get fuel injectors in there cleaney. Helps break down a shock front a few other factors. Beyond my understanding, but not beyond my fascination.