With darling wife out at the cinema today for a friend's birthday, I had the boys. Not one to waste an opportunity or resources, I put the little blighters to work! E is still a little young (and just coming down with german measles/rubella) so wasn't as much help as I'd have liked, but C put his back into it like a good 'un :-
Between us we played our parts well (E didn't injure himself, C mostly followed instructions and I kept my temper!) and made two almost square-cornered 2-foot by 4-foot frames of 1" by 1/2" pine with a 3mm hardboard cover. We quit while we were ahead so, another day, I'll coat the hardboard in spray-mount and then attach a bog-standard GW grass mat I was given a couple of years back (which I only unwrapped for the first time last week!).
Anyway, for now this is what we have (snazzy underside view coming up):
If anyone has any pointers for the last stage, or for weathering a grass mat so it doesn't look entirely like a bowling green, please do leave a comment.