Variable signup cost, donate style

Hi, I’m a pro user, I use S2 for a few sites but with one I just want to have the customer chose the amount they pay. I’m really quite stuck. I’ve considered that they take out several memberships at £1. The PayPal donate doesn’t seem to be available. I tried the only tutorial for that on here but that doesn’t work.
Any ideas would be gratefully received. Thanks.

We created our own solution for this by creating our own form that submits to PayPal. We reverse-engineered the button code and added a drop-down for the price.