Block-off plate screws/nails? into masonry firebox walls?

[19glendale]

I've built a sheet metal block-off plate for my new setup with a freestanding century s244 in my small masonry fireplace firebox. What fasteners (screws/nails/etc.) are recommended to attach the 1 inch semi-perpendicular 'lips' of the plate to the walls of the masonry firebox?

I realize this probably isnt a major issue so long as I dont use plastic or wood pegs, but would appreciate any suggestions.

 

[begreen]

Tapcons.

 

[JA600L]

And a masonry drill bit.

 

[Hogwildz]

If it wedges in there snuggly, you can silicone the entire perimeter edge and it won't go anywhere. If needed to screw, yes masonry bit & tapcons.