I've had some good advice emailing the guys at www.hairthickener.co.uk they seem knowledgeable and have been around for years..but not many good sites these days. Forums sort of lost there status when facebook started up. Same for alot of things car forums etc.
I've looked at this because somebody told me about it. I don't like the look of it...There was a cream around years ago for alopecia that was very similar and it was very messy. Such things as the Maxi professional topical shaders, and other products like this are far superior in my opinion and...
Best one I've used is Maxi fibres, reasonably priced and seems to do the job well. I think its advised you use the hairspray afterwards to lock the fibres in place.
https://www.hairthickener.co.uk/products/new-maxi-hair-thickening-fibres
Hair fibers such as Toppik and Maxi Pro Fibres can all have a good effect but alot is always going to depend what you are working with to what the end result looks like.