With a little ingenuity you can build your own extension tube out of PVC, an
old mount and some glue. :)

I'd save for the macro, Sigma and Tamron have 100mm (Tamron is a 90mm)
that are highly rated. The Canon is probably better but not by much.
Here's the problem;
For your 300mm you'd have to have an extension of 300mm or 12 inches to
reach 1:1, 6 inches to reach 1:2. I wouldn't do this on a heavy lens
with glued PVC tubing, the problem with that is that the interior of
the barrell has to be matte black or you get all sorts of flare. For
the zoom, it take proper lens design to get a zoom lens into the macro
range 1:3 mag or larger. Nikon is the only company selling a real macro
zoom for this reason (the 70-180 f4.5-5.6 lens about $800). Some zooms
say they are macro, but performance says otherwise.
If you can affords it get the Canon 100 macro, if you want one before
that the Sigma or Tamron are decent ( I don't say this about Sigma
lenses very often).

Tom

I use the 500D closeup lens on a 70-200 and with 2x TC and it's pretty
neat. Closeups at that length are more likely skittish bugs & such, a
'real' macro will give better results if you don't need that working
distance.