One of the few documented ways of making hand holds for inddor climbing is presented in this build with a unique style. It resulted in a real close to commercial climbing holds which uses the Bondo Fiberglass resin and sand as shown in the below image.
The materials that would be needed to complete the hand holds include a block of clay, Bondo Fiberglass Resin, extra Fiberglass resin fixer, play sand or fine silica sand, waashers, mason drill, and a can of WD40.
To make it easier to start, a commercial hold is cloned using a block of clay. A slice off of the clay is cut deep enough to accommodate the depth of the hold. With the back of the hold facing up, the hold is pushed down into the clay. The texture should be transferred to the clay mold before removing the hold from the clay. To prevent the poured mixture from sticking to the mold, the finished mold is sprayed with WD40 that acts as an agent as shown below.
Mixing up the Bondo resin is the fun part which should be done in a well ventilated area. The entire mold is filled up with sand and to determine how much of the mix is needed to fill the mold, the volume of sand is measured in ounces. Normally 60% sand and 40% Bondo mix are used. Before adding the sand, the fixing agent and the resin is mixed one drop at a time. To calculate the right amount of fixer to add, it is important to measure the volume of resin carefully. The holds will set up much faster with more fixer.
As shown in the below photo, acrylic paint can be used to add some color to the holds. To avoid getting clumps or pockets of dry sand, the sand should be stirred while being poured. The mix allows getting the sand grains to get wet.
The mix is poured up to the top of the mold filling it to the brim as shown below. The holds are set to cool for 1 to 3 hours depending on the added amount of fixer. When the top is hard but the hold is still a bit warm, it is ready. The texture might get damaged if it was pulled out too early. Any irregularities can be cut off using a knife and do some scraping to smooth off the back.
The next image shows how the holds are taken to a drill press. They are drilled down past the washer and can be bolted on the wall once they get real hard.