My bark physrep is made of latex - I attacked a tree with some moulding plasticene (normal stuff kills latex, get yours from an art shop or ebay), folded up the edges to make a mould, added liquid latex mixed about 60/40 with Airfix-brand paint (I'm using their World War II RAF acrylic set, don't use enamel paints whatever you do!) and left it to dry. Once dry I cut off the manky bits around the edge and it was ready to go!
Mark 1 prosthetic was exactly as described above. Far too large, slightly manky on the top surface (bubbles &c) and roughly khaki in colour. Drybrushed brown. Actually worked nicely. Wore this as a backup at E2 when one of my mark-3s disappeared mysteriously.
Mark 2 prosthetic I found a really interesting bit of bark with a lot of vertical detail and took a cast of that in plasticene, and used multiple thin layers of 30% paint 70% latex. End result was an absolutely beautiful cast that was the shape of the outside of a tree - it can be everted into something that might fit my jawline, but it can't be worn on flat areas of skin.
Mark 3 prosthetic used less interesting bark and a smaller area, multiple thin layers of 30% paint 70% latex followed by one thicker layer. End result was what I was wearing in the field: olive-drab coloured scabby barklike things which looked somewhere between horrible growths and mud-covered scabs. Don't actually look that much like bark. Prosaide stuck them quite handily to the backs of my hands (haha). These were what I was wearing at E1 and E2.
Mark 4 prosthetic used a stick pressed into a plasticene mould in a pattern, and attempted to use multiple colours of latex for the first couple of layers. Once I got bored of adding layers to the mould, I took the casting out, painted a thin layer of latex on the back and stuck it to the back of my hand until it dried - this produced a prosthetic with a very nice back that attaches very well to skin. Shame that the end result looked so poor that it couldn't be used, even after painting. Airfix dark brown paint dries too light to look like bark.
Mark 5 prosthetic started with a new pair of mark-3s - the colour here is a mixture of dark brown and grey-green, and comes out olive-drab or thereabouts. Separately I mixed up a somewhat ruddy flesh-tone and made a series of castings of three layers of 30%-70% paint-latex into a mould with a smooth surface, to produce thin layers that look like diseased skin. I cut these roughly into strips with one smoothly curved outer edge and one slightly ragged inner edge. Attach mark-3 prosthetic to back of hand with latex and allow to dry. Mix up a dried-blood tone using dark crimson, dark brown and a drop of dark blue (on reflection, should have used black as well, as it dried bluer and lighter than it should have been despite looking great wet) and use that 30%-70% to glue bits of 'skin' onto the prosthetic, aiming for a nearly continuous strip of 'skin' around the edge and sticking onto the prosthetic in several places. Wait for this horrible assemblage to dry and then remove from back of hand. Drip a little latex carefully into gaps between 'skin' and prosthetic; dry upside down on surface resistant to drips of latex. End result - 'skin' looks like dark and spotted dead skin, 'bark' looks like the mark-3 prosthetic. After another week of drying all the colours have darkened - when I apply this, I'll need to run briar-coloured makeup up from my skin into the cracks.
Mark 6 prosthetic plan: take a 'skin' mould from a mark 5, create irregular bulge in the middle by taking knot area of found stick, wrapping in spare 'skin' from mark 5 efforts and pressing into mould. One extremely thin layer of pure latex, too thin to pool in depression. One partial layer of 5% grey-green paint 95% latex in depression. One partial layer of 50% grey-green paint 50% latex in depression. One layer of 10% flesh-tone paint 90% latex. Finish as normal prosthetic. End result - Ended up using the end of a paintbrush to make a better pustule. But dear God, Humbrol paint is opaque. 5% in the latex was enough to make a completely opaque layer. It looks like dirty skin with a really really horrible green pustule. The prosthetic will need a little careful painting to adjust the colour, but at least I matched my skin tone - #237 desert tan with the tiniest dab of #60 scarlet made something that was the same colour as foundation. The prosthetic ended up specked with black from the mould - not something I'd noticed with the bark - next job, buy white moulding plasticene, genius. Failed to get the translucency I was after - will try again with three or four min-thickness layers of pure latex before the grey-green.
Space for mark 6b results.
Mark 7 prosthetic plan: take mark 6 mould, insert genuine thorn from rose cutting, use thorn instead of briar-coloured latex.