My personal opinion would be to use a square or rectangular box as that is the shape of the pig. I think it would be hard for them and a bit uncomfortable for them to use a triangular one. I may be wrong, this is just what I think. I don't have any pigs yet (Soon though! ) but I am using square and rectangular ones. I don't know though, it would be best to get others opinions, people who actually have pigs and have experiance with the triangle litter boxes. Here is a thread that asks practically the same question: https://www.guineapigcages.com/forum/about-cages/26154-do-piggies-really-use-corner-potties.html
For bedding I like to use Carfresh, especially the white kind so you can easily spot check. (I used to use this for my other small animals) Now since I can't get Carefresh anymore I get Kaytee Fresh Envirenment Bedding and Litter. I don't know what is available over there in the UK though. I also use shredded paper for (my small animal's nesting material) and over top of the litter boxes for the future pigs. I just have to say definetely DON'T use Cedar or Pine shavings, not at all good for piggies. There are a lot of beddings you may use, just ask anyone if a particular bedding you had in mind is ok. Another bedding/litter you "could" use is is KILN DRIED Pine, and I think you have to open the bag and air it out for a month? or two. Something I have to point out which is kinda obvious, but I've seen people do it. Is do not use sea shells for bedding/litter. I have no idea what gave them that idea, but I've seen it before. So it is mainly up to you and what you have available over there, and again you can just ask about a particular bedding/litter and I'm sure someone could help you out.
I have a triangle litter tray for my too piggies, they dont really use it but my itsy does tend to use it more, I put sawdust in it with a bit of hay on top to make it soak up more. you can get sawdust from any petstore here in the UK. The litter tray it ok for them to use as its has room to move but all they do is sit not move around in it.
Any way hope I helped you.