Over the course of the past few days I've had several requests for 2 1/2" by 2 1/2" plates. I mentioned this coincidence to a customer and he replied that this is the format size for Holga custom collodion backs made by Holgamods. Mystery solved!
I contacted Randy at Holgamods yesterday and had a great conversation about all things Holga and dry plates. Randy's a great guy who should be familiar to all the Holga shooters out there in the world. Works super hard supplying modified Holgas and accessories and really enjoys what he's doing. He was pretty excited about what I'm doing as well, so I expect he'll give a heads-up to the Holga community about the availability of dry plates compatible with his plate backs.
While it's not listed in the "stock list", I can make dry plates in this size just like any other size. I've been quoting $13 + S&H for a pack of 10 plates. Due to the nature of the 3D printing process, and until I have a better feel for dimensional tolerances of the holders, please measure the plate clearance dimensions on your holder so I can ensure the plates will fit. So far, measurements range from 61x61mm to 64 x 64 mm (61mm = 2.40" and 64mm = 2.52"). I'll remind you to make the measurements when you request the order.
Visit http://www.holgamods.com and follow the link for "3D Stuff" to find the Holgamods plate holding back. He also has a panoramic format plate holding back on that page.
Very cool stuff!