Glue, silicon, etc will all have an effect o the heating element. Ultimately you'll lose your glass part on the motorway one day (never happened to me, twice).
Best advice is to keep your eyes peeled on eBay. I got a genuine drivers side mirror for £11 last year. Then a few weeks later I got a non OEM passenger side mirror for £9. That's for the full mirror glass, heated backing and the two small wires. There are some high quality pattern parts on eBay, I'll try and find the link to the ones I bought.