I'd be tempted to find some nicer hardware and keep doing what you are doing. However, I assume that is putting a lot of stress on the other lug.

I would try taking a piece of stainless strap cut from sheet and loop it under the top tube against the lug and back to the seat lugs. Drill holes in the stainless to align with the lugs and run a nice bolt through the stainless and lugs. You will need to massage the strap over a mandrel to take the proper twist and shape to sit below the top tube nicely. You can polish the stainless to look like chrome.
I've done a lot of this making custom brackets and it's not hard. Might take a couple tries to get right.