Custom GUI on entity selection

[ILMTitan]

I have a custom entity. It is currently defined as a storage-tank, but it could probably be anything with a fluidbox. I want the player to be able to select it and bring up a custom GUI allowing them to change two numbers that would relate to that specific entity only. I could pop up the GUI when the entity is built, but I would like the player to be able to modify the numbers without re-plopping the entity. Is there a trick to do that that I haven't noticed, or can it not be done?

[ILMTitan]

Ended up using Wrench