It looks like an acrylic crystal with tension ring. I have not handled / inspected this particular model from Eddie’s, but I would have thought a Sternkreuz domed crystal ATC type with chrome tension ring would do the job (available from CousinsUK or HS Walsh for best price). They do not require any gasket, the tension ring “pushes” the acrylic against the case creating a watertight seal.
Happy to help if you get stuck.