If the unit is a stand alone that does not need to be level with neighbouring units then the neatest solution is to prop it in position using wedges to level it off. Then using a piece of wood at least as thick as the largest gap use it to scribe the bottom edges of the unit. Finally trim off the excess with whatever your favourite method, myself I would use a router.

An easier but less neat approach is to use UPVC trim which is quite inexpensive if you can source it from a UPVC supplier rather than the sheds. Mitre the corners attach with double sided tape and finish off and seal with silicone.

Richard