I'm moving a couple of roses from temporary pots to big, square, 'bottomless' planters, and am contemplating liners to help extend the lifespan a little. Any advice? Effectively, as I will only be lining the sides, the roses will be growing in sort of raised beds.

I do understand I will need to replace them at some point down the road as they are wooden, but tbh, almost any sort of planter would have the same issue, except the wallet hammeringly expensive modern stone composite materials maybe - and I cannot afford those, and certainly not the custom manufacture I'd need to have them as a bottomless planter.

Any thoughts? Experience? The advantage to selecting the bottomless and square self construct variety seems to me that when I do need to replace them, I can just build the new one around the old.
Also, they are pressure treated - hopefully this should not damage the roses roots?

I am contemplating staining the exterior - again, is this something to worry about?
I know the cheaper stuff I used on the shed one year badly upset those plants that got spattered, which was a lesson learned!