The forms are pretty maxed out at 2.25” firebrick, so 2” will be a bit tougher – but can be done. You need to keep the mortar joints tight / thin as possible, so here’s what I would do…
Grab a Sharpie and a 3’ level. Place the level on bottom left ridge… and bottom right ridge - draw a straight line. Continue up the oven marking horizontal lines from ridge to ridge. Do this on the FRONT and BACK of your form. This is to keep the brick joints at the same height horizontally when mortaring your bricks into place (esthetics)
Now grab some sandpaper (100 grit) and sand down those ridges. Your 2" firebrick isn’t going to benefit from the ridges in the foam… but you still get the perfect firebrick arch using the foam form when mortaring your firebrick in to place. Use the Sharpie lines on the foam to make sure your mortar joints are lining-up perfectly / horizontally.
ALSO… you can mortar your firebrick VERTICALLY if you want! Instead of having a 4.5" thick oven… your oven would be 2". And many, many ovens on the market are 2" thick. In fact, probably the majority. But 4.5" thick ovens hold their heat much, much, much longer. If you plan on using your oven for 2 - 3 hours / weekend pizza warrior, than 2" is plenty. If you plan on using your oven all day (looking at you bread bakers), then you would want the 4.5" thick oven.
If you are ordering firebricks and want to make sure you get 9" x 4.5" x 2.5" firebricks, you can order from your local HarbisonWalker. They cost a bit more - but they are the correct size and you can have the bricks delivered to your home via Residential Freight Delivery with a Liftgate. But again - try to find the bricks locally first (this way, you’ll save major $$$).
HARBISON WALKER -