An old Graflex #1234 4x5 film pack holder can be easily modified into a very nice tintype aluminum plate holder. It also works well for glass plates/ambrotypes.
The holder was originally designed for 4x5 sheet film packs, no longer available from any manufacturer. The holders are available fairly inexpensively on Ebay. The holder fits standard 4x5 Graflok-style camera backs.
The modification involves lining the inner surface of the pack holder with thin plastic, cut to fit the opening and glued in place with silicone sealant/glue. The wet plate sits against this liner. We used the thin plastic from a colored transparent file folder, cut with a Xacto knife on a cutting mat. The liner prevents a chemical reaction with the aluminum of the holder. It also helps keep plate the same distance from lens to ground glass as did the old film packs, for accurate focusing.
Thin plastic strips are then glued with the same silicone cement to the liner itself. These are arranged to allow a 4x5 plate to fit centered within the opening and to prevent the plate from sliding out of position during handling. We used carbon-fiber flat strips from a hobby shop. These are used to reinforce RC model airplanes.
The metal springs on the hinged back must be bent upward to press the edges of the plate firmly against the opening when the hinged back is closed. We have encountered two styles of the #1234 holder, one with the dual-springs (shown) and another with one large central spring. Either type works well.
One final modification is to hot-glue some dark-colored foam along the slot at the edge of the back, where the paper tabs from the old film packs protruded. This prevents any light leaks. We also glued a strip of black plastic from a take-out food container against the two prongs of the lateral spring of the holder as a further protection against light leaks from this source.
We have had one holder fail where the silver nitrate from the plate corroded through the spot welds that hold the hinge. We repaired it with some small "pop" rivets. It would be good practice to flush this area with water after a day of shooting. We also wipe the plastic liner with alcohol and a cotton pad between plates to remove contamination that can cause "oyster" defects in the developed and processed plate.