Has any one ever made a tiered rectangular cake using sheet pan? I am considering using a 11x15 for the base and a 9x13 for the second tier and cutting a 7 by 11 for the top tier. But I want separated tiers. Does anyone know how to do that/ I was thinking that I saw disposable pillars somewhere that you glued on to cake boards but I could not find any with a search.
If I can find no other way what do you think about using doubled boards or a foam core board with spiked pillars (from wilton) either super glued or hot glued to the plate would work? Of course I would not transport it assembled.
I figured that I would need 6 pillars for the 9x13 but I'm having 2nd thoughts. You only use 4 pillrs for a 14" round tier so why would a 9x13 rectangle need more than that?
Thanks all for your thoughts on this!

I don't see why you couldn't use a 9" square or round separator plate with spike (push-in) pillars. Quarter sheet cardboards are about 9 1/2 x 14 and that would sit on a smaller separator plate. You should double the cake board for stability however. Then do the same for the top tier, using a smaller separator plate. I've never tried this but think it would work. If you have two plates of each size, then you could use the pillars that snap together between the two plates. You'd have to make sure you had the proper dowells in the cakes for support. I have done a half sheet before with a small 6" tier on pillars and it worked fine.

Thanks! sorry for the delay, (family illness -everybodies ok now)
I was worried that being long the cake might sag in the middle and/or dip on the edges if I used say a 10" plate under a x13 but what you said makes sense, after all the round separators ar at least an inch and a half form the edge of the plate.
Sometimes my brain gets stuck on little bumps and treats them like mountains!