Summary

Print your own speed square holster, just like Tom Silva's from Ask This Old House.
This fits my Stanley Speed Square. It hangs over the top about a half inch just like Tom's.
Belt clip can be found in the instructions. There is a recess built in to accommodate it.

Instructions

I printed at .20mm, 2 shells, with 16% infill. Print with supports, no raft, with the belt clip indentation facing up. Be sure to pre-drill the belt clip screw hole, as the infill will tend to guide in the screw crooked as you tighten.
The model does not have the "Ask This Old House" indentation.
I super-glued the belt clip to the holster, and used a #6x1" flat head wood screw with some glue around the threads.

The clip I ordered is on ebay, 2 for $6.
"Two (2) Spring Steel Metal IWB Tuckable Hybrid Holster Belt Clips - Black" Link:
http://www.ebay.com/itm/Two-2-Spring-Steel-Metal-IWB-Tuckable-Hybrid-Holster-Belt-Clips-Black-/271763059046?pt=LH_DefaultDomain_0&hash=item3f46573166

Speed square holster