Summary

Two of these brackets mounted on the side boards will support an axle capable of holding 5kg of filament.

Instructions

Print with plenty of infill to make it strong enough to support the weight.
Position the bracket on the board as far back as it will go and mark the position of the two drill holes.
Drill with a 3.3 mm bit and then use a 4 mm tap to cut a thread.
Fix the brackets with 4 x M4 20 mm bolts and star washers.
I had an old piece of acrylic pipe spare but any strong dowel or tubing that fits through the spool hubs will do as the axle.

Reel support bracket for Mendel90