by woodbutcherbower » Thu Mar 19, 2020 8:26 pm
I carpeted mine directly onto a ply-lined floor. Best route is to use carpet tiles, they're incredibly durable, very easy to lay, and dirt cheap - I did my whole floor for around £30 as I recall. You'll find a decent selection at B&Q or similar. They can be cut using a Stanley knife and the rubber backing also makes them waterproof and easy to keep clean.
If you want something a bit springier, you can pre-line the floor with 5mm foam laminate flooring underlay. Everything can be fixed down using aerosol spray contact adhesive, you can get this from Wilko or similar. For awkward shapes around wheel arches etc. I just knocked up some simple templates from Screwfix lining paper.
Hope that helps.