Make sure it isn't very large. Adult spiders like their hides snug just as much as younger spiders. A half round piece of bark, half a terracotta flowerpot or plastic one. There are half log hides as well, and a lot of premade ceramic/stone/wood ones.
Whichever you choose, make sure it is half buried in the substrate, and that there is room for burrowing under. Don't buy something that is round or with a bottom that prevents the spider from burrowing down.
Anything is possible really, it depends on how you would like the enclosure to look .
I use a lot of cork bark, but I also have several containers with PVC elbows that you can get at a hardware store. Aesthetically speaking, not as nice looking in the natural look, but they are functional.
I scavenged some bark from a fallen tree, soaked it in hot water for a few minutes, then baked it in the oven at 350° until it was dried out. I use pieces of that and hot glue them into half-round hides. I add moss and plants as well.
These are in predominantly dry enclosures for arid species. For spiders requiring more moisture, I'd prefer cork bark like most other keepers. Otherwise, this saves me a significant amount of money on my arid enclosures.
If you go to Home Depot,these are in the garden section.Fiber material,all natural and organic to boot.
Just don't get them real wet with spray or misting.Easy to cut in half with scissors.And they are cheap
on top of that.They are seedling starter pots.
I use all kinds of things. My arboreals get cork rounds (for a treehouse setup) or halves.
My terrestrials get whatever I can find around the house that is not too hard, heavy, or sharp. (Heavy should be avoided in case the tarantula undermines the hide while burrowing.)
My husband had a couple of extra "waste" caps cut off from pipe flashing, which I used as hides for my pulchras. (I like that they are soft, light, and have a sloped shape.) They are too expensive to buy just for this use, but if you know anyone who does metal roofing work, these pieces would ordinarily be thrown away.
I have also used these silicone candy molds as hides and water dishes for slings > 1". (The part that I cut off to leave the hide open at the back can be used as a sloping water dish for slings.)