We have portakabins, they replaced the old ones last year and these are great, 8 cubicles for men, six for the ladies (cant say I have ever seen a lady enter or leave) and 2 for disabled.
There is a loo, basin, mirrors and I took an old radio in, usually have it to myself but the summer cyclists are out now so its quite busy.
I reckon I could shower before leavign and be fine, I dont really sweat much unless going for it.