I'm making this solar kiln.

https://pubs.ext.vt.edu/420/420-030/420-030_pdf.pdf

My kiln is smaller, 8' X 4'. I haven't done much framing in my life, so I'm a little at sea, although it looks pretty straightforward. But I do have two questions.

On page 9 of these plans -- North wall frame and doors -- the plans call for a 2x8 across the frame. I have two questions. 1) Since I am making a smaller version, will a 2 X 6 do for this piece? (I have one handy. That's why I'm asking. And 2), are there two 2X8s in this plan fastened together. The reason I'm wondering is that if there is just 1, I'm not sure how the doors close flush.

Sorry for the rookie questions, but I'm trying to avoid getting bogged down with mistakes.

Any other advice appreciated as well.

Thanks,

Mike