Some handles for the typical metric Allen (hex) keys sizes. The slot is tight but the keys is also held in place with an M3 screw. I used M3 x 12mm but you could go up to M3 x 16mm.

Available sizes: 1.5mm, 2.0mm, 2.5mm, 3.0mm

Work in progress: 3.5mm, 4.0mm, 4.5mm, 5.0mm

Update 2020-04-19

• adjusted key fit and uploaded 3.0mm handle

Notes: 

I used 3 perimeters and a higher infill for a little more weight in the hand and maybe more strength. standard settings would be fine.