Hi Malcolm,
External antennas under the cockpit floor is good. Remember that the antennas need a metal ground plane. I presume that area is made of metal? If not you’d need to install a thin aluminium sheet inside the outer skin on which the antennas should be mounted. This can be two separate sheets (one for each antenna) or combine both into one. Size isn’t critical, but I’d suggest somewhere around 100 to 150mm square as a minimum for a single antenna or equivalent scaled up for two. (Bigger is better).
The other basic premise is to try to keep antennas as far as practicable away from any other large chunks of metal as signals can’t pass through it, so if the radius rods are the silver bits (sorry not an expert on Moth Speak), I’d try for a point midway between the top end of those and the top of what I’d call the main undercarriage ‘legs’. Nearer the nose shouldn’t be a problem - though the closer you get to the radius rods the greater the potential obscuration due to the percentage of the antenna they will block. Also try to keep the antennas as far apart as possible but still on the flat bodywork where they still both point vertically down.
Hope that helps.
Regards
Peter