This is all very unlike the Trust and how it is operated historically. I am particularly surprised that a £500 donation appears to have been made from Trust funds without prior consultation with the membership. I can only assume that this was agreed by the Trust Board. Personally, I would favour a blue plaque-perhaps in the town at the front of the Baths.
In my opinion they ought to ask those who donate to the appeal what they want. Though I know that may not be possible due to some anonymous donors no doubt. In which case, ask the membership. Two or three options would probably suffice.