Hi Diane , cool another 6 inch refractor owner in the ranks , great scopes aint they ? , so anyway if as astro744 says just make the inside of the filter smaller by sticking that 'Sticky back' draft stopper foam from Bunnings , it comes in rolls and is about 10mm wide and 6-10mm thick , just stick it on the inside of the filter until its a nice snug fit on the dew shield .
Please do as MattT says and stick it down with a few strips of duct tape as well as the last thing you need is that the filter gets knocked/blown off accidently , safety first when viewing the sun
.
One more thing , be sure to keep both caps firmly on the finder scope ,,
I have used film , 1000 oaks glass filters in the past and both are very good but now have a Lunt Hershell wedge , this is the bees knees , but very expensive .
Here is a mate enjoying the past partial solar eclipse .
Brian.