Page 25 Modeling a Tank: Thingers

BooleanDifference the base object to the extrusion objects.

So we have 2 joined up objects and the extrusion parts need filleting.

Filletedge the ends of the extrusion objects shown selected below with a 'Radius' of 4.

Select the curve shown highlighted below and hit Extrude 'Cap=Yes' a distance of 260.

Save out the thinger.3dm file and hit SelSrf, SelPolySrf and Export Clipboard.