A standard pug wet cell battery will fit in any of those boxes Spiky (Tweeks or Magnum) as you think you may be changing it later down the line it makes sense to go for the cheapest. I think I recall getting AlexF and Pon one for around £20 each.

Si you would need to know the size of the battery to be sure of getting a box to fit. The gel racing batterys we use dont actually need boxes, they can even be fitted upside down if you like. I may know of some secondhand ones going cheapish soon.