I’m not aware of any such frame that can be bought off the shelf so to speak but it should be possible to contact a specialist uPVC company (window/door supplier etc….) that should be able to construct one for you.
Alternatively, I guess you could construct one yourself using plastic planks that can then be glued in the corners to construct the frame, if you feel confident that you have the DIY skills. If not, then you are best off asking a specialist to make one for you.
In most cases, the majority of timber ledge and brace doors and frames have to be constructed to order, to fill the hole they are required to thus this is pretty much the case for their plastic variants.
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