Been out enjoying my new Trophy and on a visit to the local B&Q (that's a DIY store to our members outside the UK) I came across and bought a pack of 10 vinyl self adhesive (12inch x 12 inch 30cm x30cm) floor tiles for £6. Having complained already about the filth thrown up the back of the bike I realised that these might just provide a simple, but effective solution. They're a tough silk finish and match the plastic on the TTSE very well.
I simply stuck 2 together for strength, removed the license plate, carefully measured up the size needed, cut and modified a cardboard template until happy, and using the two predrilled holes screwed it on. It was long enough to tuck neatly behind the plastic lip under the existing mudguard. See photo. One small hole and plastic nut & bolt was needed central and lower down for stability (hidden behind the licence plate) and lastly some Vaseline over steel screws I used to protect from the weather.