I normally roll a rag into a tight ball to fit into the combustion chamber with the head sitting on a soft surface (carpet or newspaper) .... that way nothing gets marked or damaged ....... then use a socket (a bit smaller than the retainer) and extension fitted to a "T" wrench . All you do is push down strongly on the retainer a few times and the collets pop out inside the socket (they can't fly out anywhere).
But getting them back in is a real PITA ......... you have to use CRF's ring spanner method , tweezers , a small screwdriver , safety goggles , be in a bare room with white floors so you can find ejected collets easily , and have 8 arms plus tie a pad over your mouth so that nobody can hear you yelling and cursing .....
You'd need to make a pipe with a "T" handle ....... then grind the sides of the pipe away leaving a ring on the end where it contacts the retainer to allow you to put the collets back in from the side ........... like how the end of this compressor is shaped :
http://i22.ebayimg.com/05/i/000/c9/bf/a047_1.JPG
XR75's have heaps stronger springs than horizontal engines but I was doing them on my own within seconds from age 14 without ever needing the proper tool ........ but horizontal engine heads are a totally different story because it's hard to get at things ......
I finally bought the proper valve spring compressor PLUS a few extras (due to traumatic memories)

.......
http://images.auctionworks.com/hi/52/51651/l-23300.jpg http://i11.ebayimg.com/02/i/000/8e/56/0c63_1_b.JPG
I put links to pics up like this because we've had trojans trying to sneak in ever since signing up to photo bucket .......