I'm confident that if you were to use decent wires to your meter, and used full tabbing, you would easily get your expected 8amps.
6.7A with those leads and what looks to be single tab connection is very good. (use short bits of heavy wire from panel into the meter sockets to check this quickly if you wish)
If you get rid of all resistance points (thin wire, single tab wire connections etc), you will do very well. Window glass will only degrade the expected behind glass result by about 10 percent..... much less than poor leads contribute.
Tomw is right. In this case, window glass will do fine as far as results are concerned. Hail damage is another matter, and can only be addressed on a regional basis perhaps.
Iron free glass versus normal glass is only a 5-10% type of problem.... only relevant if you have competitors. If you can source thin toughened glass it would be a good substitute.... otherwise window glass will work fine. just a touch more fragile. Thickness also counts. Thicker= more loss.... thinner is better loss wise, but more fragile... just another compromise for you to ponder.
Looks like success is close.
......................oztules