I have done this before with a 8x10"holder, but this time I tried modifying a 4x5" holder.
You can go up to 8.4 mm x 11,4mm
BE AWARE !
There are several 4x5"plateholders available.
Make sure you use the more luxury versions.
They have a massive alluminium plate inside it instead of 2 thin plates.
The cheaper versions with those 2 thin plates just don't work and will cause
a lot of trouble, like contamination, when used with wet plate.
This used Fidelity Elite is the good version.
Use a file to smoothen the cutted pieces and drill little holes in each corner.
The drill measures 1.5 mm
Use 99,9 pure silver wire for the corners and glue them with a rapid glue.
The glue is needed to make sure the silver wire is much more solid and
to be sure it doesn't wiggle anymore.
Don't forget to protect the cutted alluminium with a strong paint.
I have used nail varnish which is also available in black. It's very solid and dries fast.
Et Voila, you have made yourself a perfect usable wet plate holder!