Originally Posted by
heavyaslead
That looks to 1/8th inch plate, if so you could bolt them together with angle aluminum stock and the thickness would allow for countersunk bevel head bolts such that a bit of Bondo over the bolt head and light sanding would make a smooth outside surface. This is the method I used making my all aluminum Maus with aluminum sheet stock.
If you have a good drill press with stops, makes this method not so bad.
The angle stock is a great suggestion, but won't work along the longest seam I need to join, as the bearings for the swing arms ride right at the level of the lower plate. I'd have to do multiple, small pieces, and might as well just use angle brackets at that point. But it would be a good excuse to get a drill press