I have a period plastic tank that I used for many years on a TY175. It was intended for the mini MXers of the day but fitted the TY175 well. It had two spigots for fuel lines to be attached to (no tap). I wanted the RH outlet blanked off because it was very close to the exhaust pipe and I used an in-line tap connected to the LH spigot. To blank the RH spigot, I cut out the RH brass spigot, ending up with a 12mm round hole and fitted a (custom-made) PVC bung in the hole (interference fit). It worked OK. I imagine it would be possible to make a fitting from PVC or HDPE that would allow a standard 12mm fuel tap to screw in, and also be an interference fit in your grommet hole.