fireplace insert converted to freestanding

The SM fitting your looking for is called a "square to round". It doesn't matter if the fitting base is rectangle or square as they both use the same name. They are only made as a point of use fitting meaning you will need to have one made for your specific project at just about any SM shop.

To see if the fitting was made correct, square the base on a flat surface then eye ball the round collar. If the collar dips or is out of round then the fitting was not made correct and is useless for your project. For some reason a square to round is one of the harder fittings to make for the average SM worker, when in fact there fairly simple to make.

Your also changing the UL listing for your insert and you will void any insurance colections on a fire from such use. Wood stoves and swimming pools are the two most insurance regulated items for a home owner and are watched closely.

T_Bone

Any good sheetmetal shop should be be able to knock out an adaptor for you - its basic ductwork stuff. You probably want to use plain steel (and paint it) or stainless (painted or unpainted).

Finally, make sure that the "crimped" end always points down - you don't want creosote / water / other crud to leak out on the stove surface - crimped end down will ensure it drains properly.

Nathaniel