A plate with a peg to hang a ruler off. Or anything else that fits on a 5.5mm peg!
I modeled the base on the caliper holder by Synthetic_Future, which in turn is based on Brian.Kuos model. After trying out a couple of different Skådis-holders I really like how this base connects to the peg board and have made several different holders based on it.
I printed the model with a brim and supports just to be sure to have it stand up. It turned out great.