I smashed a piece of copper tubing down for a backer,it works great and much less mess than a free hand deal.I tried this after about 20 of them with no backer and was a 100 times better and faster.They are right ,that the side you put the copper on looks better than the welded side!!The copper acts as a heat sink too,soo you get less warpage.
If you just can not borrow or use a welder ,the screws with red locktite from the back and then grind flush ,will be better than just slapin bondo in there and you will live much longer than smeltin lead down to use.