Welcome to the site Glad your here
As far as your tank I have seen were A memberhas had a bottom pan
made and welded into place,I have also heard a member using fiberglass,
I have had one brazes the hole in and hammer toned over the tank.
You don''t want to use bondo it wont hold up to the temp. inside the tank
and you will be back to square one again next year.
As far as the Rubber coating the bottom of the tank yes do it it can only help,
you may want to do the bottom inside of the cabinet .