draughtproofing g/house doors

I've got a cheapo 8x6 rigid plastic g/h with sliding doors but they're very draughty.
The sliding 'grooves' don't seem to keep the doors very tight against the frame/plastic, and the screws where the doors meet means there is also a gap between them.

Anybody got any ideas or quick fixes (preferably cheap ).
I did think of foam stick on stuff between the doors but dont think it would last long out in the rain. but have no idea at all for the door to frame/plastic gaps

Mine has massive holes for various reasons to do with the buffoon who built it (me) and you really need them in the summer. I've just chucked some bubble wrap over the seeds trays in the main bit of draught.
Just make sure you don't get a through wind which could pick the whole thing up (happened to mine, that's why it's wired to the fence now).