Joining firebrick together to create "slabs"

If you are like me, using expensive refractory to cast slabs is intimidating, but spending an equal or greater amount of time and energy to find alternatives seems like fun!

In that spirit I offer two ideas:
First, binding the outside edges with wire.
Nichrome wire can take heat and is easily ordered.
A shallow cut down the center of each 2.5" side should be enough to hold that wire in place once it has been tightened.

Second, "peg" joinery, using ceramic pins/nails.
They are roughly 3/4 "  long and should fit into a 1/8" hole.
Made to survive the direct blast of a kiln, they should do fine sandwiched inside two bricks

Just some ideas.

https://m.media-amazon.com/images/I/31pFX3heW5L._AC_SL1500_.jpg

I looked at your pins William and I like the idea it has potential.
I do think the ones you showed are a bit too short.
I looked around and I found these pins, around a foot long, with three per joint you could join 4 pairs of bricks.
The scary part is not being able to monitor them in service.  If they failed your roof would come tumbling down.

https://www.msesupplies.com/products/mse-pro-high-purity-alumina-al-2-o-3-rods?currency=USD&variant=40236611174458&utm_source=google&utm_medium=cpc&utm_campaign=Google%20Shopping&stkn=00e3db816675&campaignid=15502649027&adgroupid=&keyword=&device=c&gad_source=1