Paul,

I am in the process of making a spring compressor and bought a cheap 6" G-clamp (£5.00 from B & Q), and was expecting it to be a complete pain in the backside to cut & drill to end up with the same kind of thing as pictured in the link that Guppsta posted. In reality all you need is a decent hacksaw (full size, not junior) and an HSS drill bit and some 3-in-1 oil. It took me about 3-4 mins to cut though the clamp when held in a bench vice, and as long as you go fairly slowly with the drilling and use a bit of oil to stop the drill bit from overheating it goes through easily enough. I wanted 7mm dia holes so started with a 4.5mm drill bit first, then went through again with the larger drill bit.
If you do use a G-clamp cut down to make an L shape, note that a strip of timber is used in that spring compressor to space it away from the base bit of timber board.