The Miata subframe would effectively raise my battery box - I wanted to have as low a C of G as was possible
Since front subframes are designed to fit the engine (and transmission if transverse), they can be a poor match to an entirely different powertrain; the crossmember which nicely nestles near an engine sump might really get in the way of a rectangular battery box.
Swapping larger engines into Miatas is somewhat popular, so more than one company offers a tubular steel bolt-in subframe, to use stock suspension bits with no fabrication, but allow more space for a different engine.
One of them is available with no motor mounts, leaving a big clear space for a battery box. Yeah, it's a lot of parts to buy (Miata subframe with parts, aftermarket subframe) and you're still building your own frame (but a simple one without suspension mounting points).
One of the "interesting" features of vehicles using Miata subframes is that the shocks and springs - both front and rear - do not mount to the subframe, but to the body (in the stock vehicle). That means that the frame of a custom vehicle needs structural points for not just the subframes, but also for four spring/shock mounts. They're lower and further inboard than typical struts, but this is still no formula car setup.
I'm not trying to sell or discourage the idea of Miata bits - the commonly used but now old NA/NB or the more recent generations which are entirely different - but they are an option and provide some examples of the issues that car builders face.