Oven build on food truck

[Cookinwithdeg]

I'm using a precast kit from Grills n ovens.com and they've really been no help in anything, I saved a ton of money on the kit but that's literally all I got - a kit with no spec sheet, instructions etc. I had a Mason helping me but he failed so I'm left to do this on my own. I'm building the kit inside of my 1971 Superior Coach that's been converted into a food truck with a steel table that's been welded directly to the frame of the truck. I just poured a 3" slab of concrete for my foundation and am now ready to start with the insulation boards, then the oven. I bought heat stop refractory Mortar to put the kit together but am not sure how to adhere the insulation boards to the poured foundation. I contacted the company and they said that normally, that's not attached so they have no suggestions on how to do it. Should I just use the heat stop underneath to attach the insulation boards also or some kind of heat proof apoxy?

thanks in advance!!