what I would do is go to a hardware store (big box) and get some right-angle aluminum stock. I got 8ft of it for less than $5 for my marquee retainer. I just primed it with some rustoleum flat black and it fit with my cabinet's motif. You can always paint it another color if you want.
if you are doing a mosaic of marquees and want to put retainers in-between you could always double this up and form a "T" shape, but that might be a lot of waste. I didnt check into it, but the stores may have T shaped aluminum stock, or you could use the metal framework that is used for suspended tile ceilings.
Or not... whatever...

joe