Echoing others, I would highly recommend PVC pipes, even if they aren't the most aesthetically pleasing. I've also seen terra cotta pots highly recommended on almost every website, and I suspect (but don't know) that they give the beneficial bacteria a better surface to latch on to.
I started my little Kimchee off with a Petsmart cave that ran about $20. When he was tiny, he mostly just huddled close to it but rarely went inside or climbed on it since it didn't actually provide much shelter. Now that he's three times bigger, my sponge filter is massive, and my 10g tank is the wrong aspect ratio (standard instead of long), I swapped it out for some PVC pipe connectors. He seems to absolutely love them - swims through them when he's feeling playful, sleeps in them at night, etc. It seems like it gives him more things to explore, while also making a lot more space in the middle of the tank for him to just trundle around.
Whatever you get, make sure you wash it thoroughly! Soak it for a few days in dechlorinated water if you can! My understanding is that PVC is safe, as well as plain, unglazed/painted terra cotta, but everything in hardware stores seems to be covered in a thin layer of dust, and there's no telling what that's made of.
The other important bit is to sand down any sharp edges (before washing). If you make a hole in the terra cotta pot, make sure there isn't anything that could cut your lotls or any v-shaped edges that could snag a gill or a leg if they get spooked and try to hide quickly. Terra cotta is super easy to sand if it's dry (but again, wash it thoroughly if you don't want a very cloudy tank). PVC is pretty easy too. I almost missed this when I got the connectors for Kimchee... pieces with bends in them or more than two openings can actually have pretty sharp bits on the inside. They can be a royal pain to sand or file down, but I wouldn't want my lotl running into a 20-degree corner even if it isn't razor sharp.
Lotl-sized PVC connectors can be very inexpensive! I'd recommend getting a few and playing around with different configurations. This post was probably too wordy, but I'm still just super excited that Kimchee loves his new furniture as much as he does.
