My first reccomndation for camping in Scotland is be sure to leave no trace.
An answer more akin to what you were really asking is to try Loch Morlich near Aviemore. There’s wild camping options and a paid campsite if you’d prefer it. There are FC toilets for all and water sports. Aviemore is a mountain bikers wet dream in summer and… The list is endless.
Aviemore is a little further for you than Fort William but since the latter is likely to involve a journey, to and fro, through Glencoe Aviemore will be a much more pleasant drive. Glencoe can be nose to tail 10mph traffic in the summer holiday season.