What affordable material for parallels?

[Shotgun]

Given that this is a one-off, you might want to give the old school method a try. Some oak logs in a metal bucket, with a blower from an air mattress will make an excellent furnace.

 

[MrWhoopee]

I looked around for a couple, though I didn't find that one in particular. The couple I called were absurdly expensive, particularly if you couldn't fill a whole 'tray' at once.

Yes, they call it a "minimum". Costs a lot to heat that oven.

 

[Janderso]

Stan's Hot Shot 360 looks like a quality unit for a fair price. I haven't been able to justify the $800.
So far I have dinked around with the old back yard heat treat. It's worked so far and it's free. Well, there is the cost of the Oxy-Acel.