macro on a (slighty less stringent) budget

I can't afford canon/kenko extension tubes atm (ie. where you can stop down to get a bit more dof), so what about getting an m42 to EF adapter, ef mount (generic so no electrical contact) tubes/bellows, and then using old manual aperture pentax lenses which are easily picked up on ebay for a few quid?

I havn't direct experience with old lenses before, so if anyone could point out any holes in my plan....

thanks

xft.y,
Yes that is doable, and can achieve very good results.
I have an old Russian m42 bellows that I use with an old Industar 50mm lens and get great results. However I would take my 100mm macro lens any day over it!

Here is a shot taken with the M42 bellows and 50mm

A Potato Bud