Hi Snoopdogie
I take it you have removed the trim plate before.... so that does not seem to be an issue. What I gather is that you wish to secure the valve-body to keep it from moving around. The opening for the fixture can be increased to a point where the trim will still cover (and seal). What is on the other side of the wall that the fixture is mounted in??
Any blocking/bracing does not have to be pretty.... it just has to work. Any scrap material will do. Liquid Nails or any type of construction adhesive will come in handy as will shims or wood wedges.
I'll wait for the pic of the unit with the trim plate removed.
SABL