If you want your legs elongated, dry wall stilts would be the easiest choice. That appears to be what at least one of them used for their legs. For longer arms I would try using forearm crutches like these:
Keep in mind that the smaller your 'footprint' is the harder it will be to keep balanced and the more stress you will be putting on your body.
The biggest advice I can give you is to practice moving on the stilts as much as possible. Get comfortable walking like that on level ground, then if you are confident, try walking on lawns or less even grounds. Overall, walk on them as often as you can. Without practice you'll end up struggling to move and possibly get hurt. You probably will also tire yourself out walking on all fours if you aren't used to moving around like that. The more you practice how you move on the stilts the more convincing character you can create.
I would also have a handler, someone to follow you around to keep you safe and to help you when you need it, like getting drinks or helping you through doorways. if you use a mask like theirs you will undoubtedly need an extra set of eyes to avoid running into people and things.
The rest seems simple, foam, burlap or cheese cloth, and some real or fake branches if that's what you are going for.