The method I use is to shape a hold in florist foam (oasis foam) and to cast a negative in RTV silicone; this silicone is very flexible and strong, albeit expensive, and cures in around 12-18 hours. I then cast my holds using polyurethane resin; a two part liquid system which I believe is the same as/similar to the material commercial holds are made of. This resin turns out really strong and non-brittle so has less chance of breaking when tightening against a wall or climbed on.
Florist foam - I used a brand called Oasis. I tried the dry and wet foam and found the wet green foam gives a better texture
RTV silicone (room temperature vulcanizing silicone) - comes in two parts silicone + catalist
Polyurethane resin - comes in a two part liquid
Paper clip - optional i used it for stamping in to my hold
M10 bolt and washer - used for reference and making a hole through the hold
I got all of my materials apart from the paper clip from ebay and should be available all over the globe.
10mm drill bit
Cone nose pliers
Plastic mixing cups
Step 1: Creating the Hold Shape
All you need to do it cut a piece of the florist foam off your big block a little bigger than you want your final hold and roughly carve to the shape you want using a small knife. As you will find this foam is very delicate which makes it ideal for smoothing and final shaping with your fingers....it can get a little messy though.
Next you will need to create a hole through your hold big enough for your bolt. I use an 8mm auger bit to create my hole is it was something I already had, i then enlarged the hole with my 10mm fixing bolt. I then countersink a larger hole for my washer using a small grinding wheel on my hobby drill. Try to make the hole as perpendicular to the flat back edge as possible as this will reduce the chance of the hold spinning.
Step 2: Making Your Hold Truly Yours (optional)
Having come up with several ideas that night and dismissing several ideas the next day I remembered an instructable I had seen about making jewelry out of paper clips and other common items. If you haven't seen the work of Mrballeng I suggest you have a look this guy is amazing, here's a link to give you an excellent guide to bending paper clips! https://www.instructables.com/id/Butterfly-Bracelet/
So anyway i straightened the paper clip and using cone nose pliers to make nice even curves I bent the moniker I wanted and pressed it in to my hold. I do need to look at a good way of doing this as some of the intricacy is lost when pressing on a curved surface. Maybe I'll try gluing it on to a curved piece of wood and kind of roll the impression on? The impression was much more subtle and sharp when I tried it on a flat piece of foam as there was little movement and the depth was uniform.
I thought it looked pretty good though.
Step 3: Making the Mould
In order to work out the volume of silicone required you can pour in some lentils as they are nice and small and will get in to most of the gaps. Pour the lentils from the forms in to a mixing cup, a cardboard or plastic cup is good, and note the height of the lentils. Next put the cup of some scales pour the silicone in to your empty cup up to the required height. You can now calculate how much of the catalyst you need. Stir the catalyst and silicone together really well until the colour is uniform and you are ready to pour. There are quite a few clips on the internet about the best way to pour silicone which seems to be fairly slowly, from quite a height and in a thin constant stream; this will get rid of most of the bubbles. All you have to do now is wait for approx 12-24 hrs. Once cured break the mould from the form and clean out the inside of your mould. As you can see from one of the photos I did get a couple of leaks so I sealed them with plasticine which seemed to stop them well enough.
Step 4: Pouring Your Hold and Admiring Your Work
I used a similar method with the lentils to measure the volume of PU required. I weighed the amount of lentils I needed to fill the hold and divided by two, I then poured this halved amount in to a small plastic shot glass; this hold used almost two whole shot glasses. I then put my paper cup on the weighing scales and poured in two shot glasses of the hardener part of the PU and noted down the weight. I mixed in my colouring making sure it was well mixed and then poured in the same weight of the second part PU. Mix this very thoroughly and pour in to you mould. All you have to do then is sit back and relax for half an hour to an hour until it is set.
Once you have waited as long as you can stand make sure the PU is set quite hard by squeezing the mould gently and if it is set turn it out and see how it came out. The first one or two may have some residue of the floral foam on the finished holds but that will soon go. I also got some 2mm thick rubber that came in an A3 size sheet that I cut to the same size as the hold and mount on the back of the hold, which should reduce the amount of spin that may occur when climbing. I haven't worked out how to stick it to the back, super glue didn't work well at all, gorrilla glue was OK, it held it on fairly well but was easy to peel off - I guess this doesn't matter though as it will be between the hold and the wall.
All I need to do next is make a load more climbing holds......oh and I guess I would need to build my wall too, but that will be a whole new instructable!