Was in Lidl today and they are selling artificial grass mats with rubber backing for £5.99 They are 200x100cm
I have used these for the last 3 years. They work quite well as a flat base.
I got some samples, from a company the name of which escapes me, a couple of years ago when I was planning to replace my real lawn with something that didn't need mowing. They were ideal for representing corn (sweet corn) in 10mm but not for anything much else.
Our glorious leader bought 4 a few months back, they tend to just sit in the cupboard, as they are a strange size and don't flatten out very well. Best use - fields, but there are cheaper ones around for that.