Waterproof seat covers?

Any advice on where to get some thick waterproof seat covers that are easy to fit and remove? I use it for shooting so I am very often wet and muddy!

I bought some Landrover ones but I find them fiddly to fit and they have lots of gaps (probably due to them being air bag compliant?) in the side and back.

I like the material (waterproof cotton I think)although it does seem a bit thin. I don't want nylon but anything else considered.

Thanks

Getting some from SB Seat covers for christmas. Same as these Car Seat Covers - Anthracite Cloth Fabric with Red Bolsters but in blue with seperate headrests. My old folks say they look like really good items so Im looking forward to trying them out.