I have a Mason 2x4 with raised flue pan:

2X4 XL - 2.jpg

It came with 1/4" insulation which I installed. I bricked the bottom of the firebox with full bricks and the sides with splits. If you look along the bottom of the rail, I cut thin pieces to fit in a 1 inch gap. I should have probably just stuffed the space with some insulation. But it works fine.

brick 1.jpg brick 2.jpg

I had to custom cut a significant number of bricks to get them to fit. I rented a wet saw and, after that didn't work, went out a bought a wet saw. Made the job a lot easier.

The arch gets hot, but you can touch it for a second and won't get burned.


Mark