What I've found to be the best tool for getting those valve locks to seat in the retainer is just a good flathead screwdriver just by pressing down on the retainer and compressing the spring with one hand and manipulating the valve locks with a small precision screwdriver, tweezers, or whathaveU with the other hand.
It works, and I haven't had a spring assembly catapult off into oblivion yet, or mar the virgin smooth surface of my cylynder head.
I've tried to modify a valve spring compresser I picked up from Sears. By removing material from the clamping arms, so that It would have more clearence, it compromised its ability to compress the spring. And even when I could get it to compress a spring enough to insert the valve locks, I would still destroy the valve stem seal...even at times when I was certain that I avoided it completely.
Pressing down on the retainer with a screwdriver is easy as long as you don't have it slip off, and I've yet to have it happen, but I have lost three vlave stem seals due to meddling and messing and trying to modify a spring compresser.
Now....I can hardly wait to club three more seals into that baby and fire it up.