I'm going to test on some other filaments I have today .

On side note i edited the instructions to include hints as to how you can adjust the scale of your part to match the wall thickness of your extrusion stock . the wall thickness on min is 1.4mm , the part was sized to print with PLA and just be a snug fit on that wall thickness .
you could just try applying % diff in the wall thickness of your stock , to print the first 3 or 4mm high of the part and abort , then when it cools check the fit of your stock in the vertical slot. the key is the thickness as there should be a small but of space at the tips of the extrusion sides. scale your part in your slicer to fit your stock perfectly .

One friend has tested heating up the extrusion before insertion into slightly undersized PLA end caps, it looked to work ok , but then you cant use the vertical at places in the middle of your extrusions very easily . so im not sure if the heating that way is a good idea.

possibly passing a heat gun on finished assembly could further solidify the parts in place . That would be something to try .

I have found the PLA i use to be real strong , but can see why one would want to try to solidify the layers.