The idea is to build a "hardpoint" in the junction between longeron and crossmember, and install an aluminum bushing (to spread the load to the spruce without having to use a heavy a 5/8" bolt). I've also doubled the cross piece (not shown yet here). I'm trying to include as much structure as possible. T-88 was used, of course, but this photo was a dry fitup. I never clamp T-88 tightly...it's better to lightly clamp it to allow the T-88 to penetrate the wood as deep as possible, rather than squeezing it out.
Here's the "assembly", but it will be epoxied in place one piece at a time, but all within a few minutes so no epoxy will intefere with another part. Total weight for both sides is 10.2 ounces.
Here are the pieces. You get the idea. This is 12 ply 1/4" Finnish plywood and a spruce block for the aluminum (6061-T6) bushing . the bushing is drilled out to fit a 5/16" bolt.