Montana 750i weather seal caps falling out. Shrunk

I have a Montana 750i and around 12 months after I purchased it I noticed the two little rubber caps were hanging out more and more after each trip. They appear to have shrunk and now with the slightest wobble fall out. I have been told by garmin aust this isn’t a replacement part even though they have a small screw next to them and look simple to change. I have been offered a refurbished one for almost $300 aud but I’m not paying that for a $3 part.Granted I should have acted sooner as it’s now over two years old and out of their warranty terms but it has cost me $1500 aud all up and now not waterproof. Anyone have any solutions without silicone them in and will never come out. 
regards.