>>1718503Poplar. Easy to work with and strong enough to take a beating. Cheap too!
If there are any lumber yards near you, go and ask for a 6' length of 8/4, S2S poplar, about 6-7" wide.
In case you're not familiar with what all the means. Rough lumber is measured in quarters of an inch, so 8/4 is eight quarters of an inch, so, 2" thick material.
S2S means surfaced 2 sides. The two faces of the board will already be planed and parallel. This isn't strictly necessary, but it'll make laying out holds easier and give you a nice flat face to use against the wall and to reference against when you drill for bolt holes. Because the rough material starts at 8/4, once it's dressed it'll be closer to 1 3/4" when you buy it.
If you get a board that's 6"-7" wide, you can easily fit two small/medium sized holds next to each other for efficiency, but it still gives you plenty of room if you want to make a bigger hold.
If you want to go crazy and make large jugs, use tightbond type 1 wood glue (or type 3 if it's going to be used outside) and laminate two pieces of 8/4 together.
The quickest way to shape holds is with a bandsaw, rotary planer attachment for an angle grinder, a 5" disk sander, an orbital sander and a drill press.
BUT, it's likely you don't have all/any of those tools.
You can get by with a jigsaw, a few rasps, sandpaper, and a hand drill. It'll just take a bit more time. But that doesn't seem to be lacking right now.
Have at it, good luck.