vexillum crossbar

[Quintus Aurelius Lepidus]

Hi

I've asked someone to make me a vexillum. They have produced one which has a sleeve on top. The crossbar needs to fit in the sleeve completely. (it'll come out on both sides but for the rest, it is inside the vexillum.)

How can I attach the crossbar to the spearshaft with this sleeve? using a nail? or making a tiny slip into the sleeve on the back ?

thanks