For completeness, here is the resulting set of brackets. It took about an hour to cut these out, drill holes, mill the slot and finish. I'm embarrassed that the slot is somehow 1 mm off center, but that doesn't matter for my application (opening heavy sliding vent windows in a solarium). Nevertheless, it won't happen again!
The dimensions are close to those shown on the product page pololu.com/product/2355. The fit to the actuator mounting stub is slightly loose and was improved by a couple of thin washers. Finally, a 1/4 inch carriage bolt makes a fine clevis pin for the generic linear actuator.
Raw material: mcmaster.com/#steel-hollow-bars/=w6k9rc (6527K73)