Regarding the solidity of this technique, these days a lot of indoor climbing holds are made of fiberglass held together by epoxy, with a thin layer of textured

Regarding the solidity of this technique, these days a lot of indoor climbing holds are made of fiberglass held together by epoxy, with a thin layer of textured plastic on the outside. These are generally called fiberglass volumes. The material is so solid that a meter-wide hold is actually just an empty shell with a thickness of a few mm. This easily withstands a heavy dude jumping onto it sideways (in fact, that's what it's there for!)