Hey Jeb....
You dont have to fill the entire hole with solder. The main thing is to make a good electrical joint with the solder being the electrical "glue".
Attach and crimp the wire to the terminal hole and heat the joint well and let the solder flow around the crimped with and terminal so they get
"Glued" together well, a nice shiney joint. When its cooled down you should be able to see a nice shiny joint with a little flux on it from the soldering process.
Dont worry about the flux or resin stuff left over on the joint....some like to take a q-tip or small bristle brush and clean with alcohol but this also can make a mess!!
One thing when I built my crack (and I have built many, many amps and circuuts in 35+ yrs!!) is to follow the instructions very carefully..being an old timer I can solder this stuff from the schematic but the items used in this kit have some small important things like upper and lower holes etc....and the crack designers have utilized these mechanical things to the utmost...to make it easier for a DIY first timer to be successful...(wire layout to help minimize noise, hum etc...).
The point I am trying to make is if you did fill in an entire teminal hole and had to later wire something to it, you have no hole!! So solder when they indicate and not before!
Enjoy you kit building experience its really neat when you turn it on for the first time and hear wonderful music!!
Post any questions here and many folks will help you!
Alex