Crossed my mind to bend the 3/16" plate (if possible) to bring the bolt into alignment with the valves. Only needs about 3 degrees, not a big bend. I think that would be enough to make this work.
But even then, if you have valve springs bigger than the 1.42" diameter Magnum ones, this tool isn't going to fit between them. Assuming something like an Edelbrock cylinder head uses a more standard diameter spring and the valves are spaced the same, there is no way you could build a tool that would work.
In the end, I think the best option might be the stud mount spring compressor. Hughes sells a stud with a 5/16 thread on the bottom and 3/8 on the top for like $6 and would allow for the tool to work. Hughes even sells the spring compressor and includes the needed stud. I like that one because it doesn't require you to keep pressure on the spring with one hand while trying to fish the keepers in or out with the other.
Second option would be the shaft mount compressor and a plate as shown above.
